If there's going to be a cameo, I think it should be a younger Scotty.
The MC see Pegg tinkering away on some transporter gizmo, and she starts chatting him up. He tells her he's working on a new/better transport re-sequence equation--less scrambled molecules. She offers to help test. So Pegg goes and stands on the transporter and she beams him out, waits a few seconds, and then rematerializes. Chris Doohan appears and they start chatting up as if nothing happened.
Pegg & CGI Doohan?