Going to post in here since I have a similar question, but no airline lounge access, but access via Priority Pass (and Plaza Premium).

Coming in on UA arrival ~6am, leaving on a separate SQ ticket in the evening, ~14 hours later. Both flights in Y, and I'm only *S, so no alliance access. Most PP lounges talk about a 3 hour limit, and needing to show a departing BP. I'd love to get a shower on arrival, and I thought I saw another thread somewhere, but can't find it, listing lounges that accepted passengers without a departing BP (or at least one within 3 hours). Since I plan to go into city, either airside or landslide lounge (which I think is just Changi lounge) would work.

Is there a list of lounges, preferably T3 airside, or landslide, where I could enter after arrival to grab a shower before leaving for the city - so in essence, one that will either accept an arrival BP, or my departing BP for farther in advance. Second question would be if I could return to that same lounge later, before my next flight - probably not a big issue, since there are a few, but if I need to avoid the one I was able to use in the morning.