IT WORKS! I CAN'T BELIEVE IT!
Errmh, well, here's the deal - only the Monk can realistically pull it off IMO, and it kicks ass in the labyrinth. Lining up Rex is a bit of a problem if you intend to level up while you're at it (he runs away to any popcorn you munch), so I decided to go with the halfling to have a load of health potions (wouldn't've worked with anything else IMO). Wheytwut would've helped but, eh, it's prep JJ or go scumm it up. Killing Rex afterwards is a pain too. But that Goo went down unreally fast. However, my 68% resists monk was having problems tanking 100% knockback hits from Rex, so I doubt anyone else has a chance (curious about warlord, though, except what might happen is rex blowing your DP and knockback killing you).
The only practical application of this that I see would be... none, really. Maybe someone can find an easier way (fat chance, this is basicaly a "prep Binlor in Labyrinth = proffit" run), but I'm not sure who the other boss would have to be to make this, very, very nich approach better than any other one. Also, anyone but the monk would have a hard time assembling the necessary resists (maybe paladin), but if you can find a fat enough Minotaur who doesn't kill you outright there could be some shifting passages hilarity up for grabs.
If anyone has a better way of going about this and a place to do it I'd be really curious about hearing it. Technically, I think I would've maybe had an easier time knocking the goo into Rex with Binlor knocback and a less gimicky build, but that goo went down so fast that I'm not sure anymore.