I actually gave him a cameo in my own Indy 5 script. It's a play on the Raiders scene where Indy talks to the government agents. This time, the disbelief comes from "Agent Sellick", who tries to talk down to Indy, saying "Yeah, yeah, you found an Ark, so what? I'd have gone after it myself if I wasn't busy surfin' in Hawaii".
Although, I like your idea better: Sellick's character could be that fallen shadowy reflection Belloq spoke of. Very cool!