Final Fantasy 6 Alternative Universe project (Latest update: 02 06 2015)

As many times as the normal game checks for imp status, I'm sure there is a way to do it in event mode. At the time I couldn't tell you how.

As far as disguises, those are normally handled (as in the Locke senerio) by changing the graphics set for that character. Doing it via a status effect would open some fun possibilities, but even that only changes the displayed graphics (the sprite sheet) after a check for the imp status. This would also require some other rewriting of vanilla events that clear all status effects, if for instant you wanted it to remain while switching party members, or resting, or many other times. (On a side note, with some effort wouldn't be impossible to adjust other status effects to change graphics set... I think).

Either way requires replacing not only the character sheet in question, but another full character sheet for the disguises (you said you were looking at replacing the kappa sheet for this case). I was working on a way to add new full sprite sheets (so adding something like that wouldn't require replacing stuff) but hit a small snag then got ovely busy with everything else. In another month or two I'll try again but untill then... this paragraph is basically useless, just thought I'd mention the possibility that, I think, it IS possible.