I've heard that when the app gives some permanent notification then it won't get killed. This is what ES File Manager is doing to ensure that copying/moving operations will always complete. I'm using it a lot on various devices (only on Kitkat and Lollipop though) and while the main app often gets killed randomly, the copying/moving subprocess always finishes and stays opened with a notification - sometimes even for a few days if I forget about dismissing its window.
Also, I only vaguely remember that there is some property of the app (or permission?) that will make the app unkillable by the system. Maybe it is only for rooted systems (via an XPosed module maybe?) but still worth a try...
You guys - as programmers - should check these out, maybe something along these lines would be the solution?
EDIT: found an Xposed module but haven't tried it yet, the reference is there:
http://androidforums.com/threads/app-killer-whitelist-app.727684/