Their textures are 6x6 by default, but may be optionally 8x8 once I add player head support.

I'd love to add the necessary APIs to make this work.

Do you have a cancelable robot render event in the buildcraft API? If so I could make a cross mod add-on for it. Would be a good test of my new skin API.

EDIT:
Or are you thinking of something that just renders over the robot not replacing it's render?

Just renders over the robot. The robot is a 6/16 x 6/16 x 6/16 cube, and then I'd probably add a way for an Item to implement IRobotOverlayItem.