Originally Posted by MSPeconomist

If you're a PP member, there *might* be a new land side PP lounge at SIN, which would require a same day boarding pass but not a departure from SIN. Check the PP website for location and other details. Otherwise AFAIK all of the pay lounges at SIN are airside and probably would not let you purchase entry without a departing boarding pass, but you might want to check their rules (I assume they're posted on some website) to be sure.