You just want something that will natively edit ABGR bmps rather than having to do conversions manually at either end?

Yes, because it's not only doing the manual conversions.

If I'm just doing rudimentary editing, for example swapping buttons around, or stealing some buttons from another app until I can pay for or some original artwork, then it's a PITA to have to do it all on pairs of bitmaps.

Obviously, if it's more intensive editing, then I'd be doing it on multiple layers, which is almost the same amount of work as multiple bitmaps.