There have been discussions here about being denied entry at the international lounges with a domestic BP. Just be mentally prepared for some pushback in case you encounter a difficult agent.
Originally Posted by
carrotjuice
However, be mindful that flights from SIN would likely arrive at G or H gates and, with the aerotrain down, you'd need to bus to C gates and then bus back.
OP asked about the Regional lounge, which does make more sense as a OWS.
Not sure what time the OP will be arriving, but the evening SIN arrivals are a mixed bag, with C gate arrivals being fairly common.