I don't see how an FO in the RHS during normal line ops should be logging ICUS. As others have mentioned many airlines and regional operators use pilot flying and pilot monitoring roles which are interchangeable between capt and fo only some specific duties which are left or right seat orientated. If a fo joins an airline flies from the RHS and carries out normal duties this is co pilot time....
I only reckon ICUS should be logged for a candidate in the LHS undergoing command training this is when they are truly In Command Under Supervision. Using it to top up hours for an ATPL just doesn't make sense to me