What happens when a Subaru WRX STi humps a Subaru Brat?

This does. The Brat body is on the receiving end of an apparent full STi engine and interior swap, although there's no word on whether the Brat's traditional ultra-hazardous jump seats in the pickup bed were preserved. It's certainly not much of a sleeper in World Rally Blue with those UK-market wheels and exposed FMIC. Plenty of pictures in the link below document the transformation.

Edit: Link is now fixed.