Version 4 of BigAtom implements the bigatom_Overlay() proc, which is a step in the right direction even if it doesn't have all the nifty features I would like it to.
There are two important things to keep in mind if you are using this early version of bigatom_Overlay():
First of all, it only works in BIGATOM_NOOVERLAYS mode. ("NOOVERLAYS" in this case refers to how the BigAtom is displayed. It forces each tile of the BigAtom to have an individual atom.) Since this is the default bigatom_mode there shouldn't be many problems with this.
The other issue is that your overlay should match the base icon in size and icon_states, or it will display the basic 32x32 mini icon on parts that do not satisfy the condition. For instance, if you have a 64x96 person and try to overlay a 64x64 shirt on them, the top left and top right icons will have mini shirt icons on them. If your shirt doesn't have a "punch" state and you flick "punch", every tile will have a mini-shirt until the flick is finished.