I've been trying to take what's in the corpse.lua file (animation with proper sizing and b1 sprite) and throw it into the destroy_action {} event with no luck.
in the unit prototypes, set the value corpse = "robot-corpse",
then change the sprite for robot-corpse in corpses.lua in the "filename" field, and set the correct width, height, frame count etc.
you may post your own mod with the re-skin but what you could do is actually just keep your own mod small and make it 'require' robot army mod, and your mod can do a 'updates-final' and grab the robot army prototype tables and inject your own filenames and paths and allocate the corpses and animation tables yourself as like an override. This means you don't need to reproduce the entire mod itself, and if any updates happen it is less likely to require you to do make all the changes/fixes that I have to make anyway.