Not sure if this is relevant but I have, on many occasions, arrived in ORD on UA with a C boarding-stub and I have gotten into the RCCs no problem upon presenting the stub. The policy is that RCCs are open to all Int'l C and F passengers, and I'm somewhat sure you can gain access upon arrival as well. Unlike, say, SQ's SKL where the lounge is only for departing passengers which means you can't use the SKL at TBIT (it's probably closed for renovations).

Give it a try