Don't you need a Command endorsement to act ICUS???
I see no reason why you can't log ICUS for sectors flown from RHS (regardless of who the Capt is), but if you only hold a Co-pilot endorsement, then you can't be "in command" whether under supervision or otherwise.
Do Rex FOs hold a Comd endorsement? I know QL FOs only have co-pilot.
The fact I consider the concept of co-pilot time/endorsements to be a hangover from years past made irrelevant by the concept of PF/PNF is besides the point.