I've been rejected multiple times at both SATS and Ambassador when trying to use an arriving boarding pass. One one of those occasions (at both lounges) I was actually arriving, and on another I was testing them out by using my arriving one even when I had a departing one. Rejected every time.

My understanding is that the lounge in Jewel does accept arriving passengers, but I've only ever used it when departing - and would avoid it again unless it was the only option available...

The last few times I've walked past the SATS lounge in T3 they have had a sign out the front saying that they are full and only allowing passengers "with a reservation", so presumable no walk-ins regardless of whether you're arriving or departing.