You will be allowed into the SK & LX lounges in T5 as a UA *G.

Also, you probably will be allowed access to T5 with a same day boarding pass. I’ve personally not tried it in ORD but wherever I have tried in the US I have got access to a non departure terminal eg: EWR, LAX etc. to access the lounge. TSA are aware of this and permit it.

Make sure you have enough time to return to your terminal. You’ll have to go through security again which depending on the time of day in ORD might not make it worth the effort. It’s a long way back to T1/T2. And neither lounges are that great.