Uuuuh, nice subject ^^
Are you interested in only those or other janky stuff / weird locker considerations? I'd hate to see it devolve into a "yeah, but x is more powerful" or "that locker slot is reserved for y cuz math".
Sensation Stone: Weeeell, I sort of broke it, as you may know, back when it gave a bigger CP boost. I sort of measure it against the Fabulous Treasure and in fact end up lockering and using it sometimes (In fact it's in my locker right now and that's a feat TBH) ^^ Thing is that sometimes the gold boost from the treasure is overkill, and the gold you get from the stone is enough and the CP amplified by a transmutation stone is also very nice. So if I want an immediate gold and CP boost with no exploration (or popcorn mopping such as from Midas Gloves) I like to take it along. There's no specific strat, but it translates into getting some extra CP and being able to pick up an item from the shop - if you have prep money to blow it's very nice and not powergamey.
Mas09 Ledger: My first reaction to it was "Dear god, this thing is unbelievably broken". Then I tried it, and it was incredibly broken. Then I gave it up and went checking other stuff out, because the way that it's powerful is sort of always the same - with a bit of RNG luck you have a strange mega wonnafyt. Saves a ton of exploration, let's you cherry pick targets, messess up the dreaded and hated blinkers and lifestealers and blinking lifestealers. Hilarious with monks, interesting with fighters. It was always on my "got to have a binge of this one day" mental list. I also suspect it's less broken than it seems, but that could still mean it's very, very powerful.
EDIT: XD XD XD

:_D OneMoreNameless I want my children to have children with your childern XD That is so very accurately what passed through my mind when I first saw it. And every other time that I saw it. EDIT: And I never for the love of god could figure out why you just get it handed to you in the subdungeon while way less powerful stuff has you jumping through hoops o.O
Fabulous Treasure: Eh... If they hadn't nerfed Tribute from 10 gold for 10 piety to 15 gold for 10 piety, this thing would be off the charts. The way it is it's a decent bucket of TT piety and it means you don't really have to worry about money when purchasing items. If you want to go moderately silly with TT it's the best prep there is, if you want to go CP heavy and do a whole shop buyout it's the best prep there is, except when sensation stone is the moderate option. I love it on vampires for the buyout, it's nice in conjunction with the Alchemy prep because you're guaranteed to pick all the potions up and I guess it's good with the elite items prep (allthough you'll still potentially run out of gold). Supreme Slayer via TT makes it a dooooh... It's a niche prep, but a fun one.
Pepper the Dog: You know what, I never ever lockered it. Can you in fact locker the other items from that challenge? I know how it works, but it's large and it's only really relevan for the priest, but I was having so much fun with Avatar's Codex priests that I never felt like lockering it.
Titan's Guitar Oh, god, the shennanigans!

I actually use this when I really want to do a vicious PQI with a specific weird strat and want to have freedom of movement (and avoid binlor). It's an alternate game mode, for sure. However, it really clues you into how restrictions aren't a bad thing - it's very easy to have no sense of direction without walls and a lot of exploration you gain by prepping it you waste. I also used it to clear all missing runs on the Labyrinth while testing out the deeper end of the PQI, since I really wanted that place out of the PQI. It's a godsend on Dragon Isles* if you want to try silly strats.
*The reason it's better on Dragon Isles and Doubledoom is that prepping it doesn't remove the basic shape of the level with destroying restrictions, it just removes the walls which are always in the middle of the "isles". This lets you move more freely whereas before you'd be stuck having to get past enemis which can be resource intensive. It also opens up the secret subdungeon for sure. If it just removes, say, all walls on a map you can really waste all the exploration that would give you, or in some cases more exploration than you gain by having no walls.
Spoon Yeah, free prep is free, what can I say - it's only real flaw is that it takes up a shop space, otherwise why not? I keep it around for a while too.
Long rant It's lockerable? Hmmmmmmmm. XD I'd prep it just so I could put "I have a Long Rant in my locker" as my signature, but it would be bad taste. Acid Caster sinergy is obvious, and +3 damage on everything is actually pretty good.
Now my contributions:
Naga Cauldron This thing is in fact good and fun, and you can play a warlord with it. I whish it worked with Thieves, too, and priests rock with it. I don't play elves very often (or at all it seems), but if you're going for an explosive mana pool and don't depend on an item prep, I think it could be good, but if you're going for an "endless health potions" build with halflings it's really nice. Pair up with RBS to give yourself slow strike, get cursed, get weakened, get corroded (JJ will do that for you), and enjoy the overheal. Bring patches along.
Crumbling Ward This is an obscure pendant which you can find in the south I think, it can be consumed for +15 piety. I find it nice, and it was much nicer when all the piety farm gods weren't all the rage.
Bound Sword +15% damage, but can't be converted. It's overshadowed by the Dwarven Gauntlets, but if it wasn't, it's probaby be in a lot of lockers.
Elven Boots Wheytwut? What's obscure about them? Well, you can find them very early in the game in the graveyard subdungeon. It's an easy way to make all your early spellcaster runs pretty breezy way before you can realisticaly unlock the elite items.
Mage Plate If I'm going spellcasting, I'm not gonna say no to +4 or 5 mana. The damage reduction doesn't apply to pissorff damage, so it can be rather good. Before I knew abot the Elven Boots thing I was happy with it, I whish elites weren't preppable...
Keg of Health Prep it on a dwarf. Just saying.
I'm sure I could come up with more, and I'm sure I could come up with particular strats, this was just comments and sort of expanding the list if possible. God, I'm so curious about what other people have come up with ^^
EDIT: Oh, Oh, one more!
Bogus Slayer Wand - Isn't there a bogus slayer wand that leaves behind a bigass Animated Armor? I'd locker that thing if I ever found it again if possible to be able to bring 2 slayer wands into a pissorff run. If you remember the time when due to a bug the regular slayer wand left an AA behind? I didn't realize it was a bug and I liked it XD If that AA isn't no-XP it would mean I get to one shot one guy, and then kill another guy, lots of fun to be had...