I remember that bothering me a while ago, and giving the problem some thought. Perhaps if detecting the exact moment of empty slimeling isn't terribly important (and indeed it can't actually be done in all situations), it could be done by checking for action when combat is won. AFAIK there are no situations where you would win a combat and not have the final round show one of the actions indicating it still has some charge left. IIRC, it's a 100% guaranteed action with two possible outcomes (starfishing or nothing), and the final combat round can't be a quick action. If you had a script to feed it whenever the fullness got too low, it would occasionally go a combat empty, which would trigger a reset. But you wouldn't be feeding it more than it used, with the current method for calculating charges from equipment. There might be some weird interaction with familiar-blocking enemies, but they're pretty rare. I think one of the weirdeaux butts might stop it from acting, though I haven't tried with a slimeling.