I found this listed as one of the privileges of RCC membership

"Reciprocal Club privileges are extended to United Red Carpet Club members when traveling on Star AllianceTM member airlines and US Airways Clubฎ."

Thus, the OP, with a boarding pass for a departing *A flight, should be able to visit a *A Gold lounges in Singapore which includes TG lounge as well as the Premier Lounge. No access to SQ's business class lounge, though.