Operating carrier's policy is what dictates. UA won't even let you fly them on that outbound - even though LX allows it, UA in operating the GRR-ORD segment would have to take responsibility beginning in GRR and hand them off to LX in ORD, which UA explicitly states they won't do - they don't allow OA for UNMRs and they don't allow connections for UNMR (interestingly they seem to spell both out in their policy, even though it's pretty much implied no OA if you don't allow connections). If you show up in GRR with this, even if LX let you book, you'll have to check in at the counter since online check in will be blocked by age, expect to be turned away by UA for a non-compliant booking.

As others have indicated, consider doing the GRR-ORD drive, or if you want, book a RT GRR-ORD for yourself to "drop off" and "pick up" in ORD.