I feel I should say that while the stuff I mentioned in that thread is possible, it was mostly "best case scenario" type stuff. Due to more recent developments (or to put it better, the lack of developments) I most likely won't be doing any of the crazier stuff that was posted about. At the moment it is looking like plan B will be the way to go, which is to wait for JJ2+ to support more than 2 exits in a level, and then just use that. Maybe in conjunction with a simple background "utility" in the vein of karrtotshire, if certain problems regarding saving and loading games can be resolved.

Interestingly, a custom anims.j2a would probably be the one thing that can actually be done, right now, with existing tools. All it requires is the skill to make your own sprites. Of course, without some more advanced hacking, your new enemies would behave (and sound) exactly like the old ones. But that's no problem, since most jj2 enemies just walk back and forth anyway.