Now that Copa is at international terminal A (i.e., not the Star Alliance side) according to the SFO info site and I am on the flight that leaves at 1:30am, I assume there is no lounge access? Please tell me I'm wrong!

Correct. No lounge access at A unless you are willing to go to the UA club at Concourse G and clear security twice.

I did this flight last week. It is amazing how sleepy SFO gets after midnight aside from the late Asia departures in G. No concessions in A are open other than the duty free shop.

As mentioned above, the only realistic lounge option is the UA Club in G, which closes at 1am and requires either a long walk behind security or doing security twice. The UA Clubs in E and F close even earlier. I initially was maybe going to try the AC MLL in T2 and walk to A afterward, but I was a little shocked to look up beforehand that it closes at 19h15.

If you're local to SFO, my advice would be to nap and take a shower at home and arrive as close to the flight as possible. The Copa flight is basically the only departure in the entire International Terminal at that hour, so there's no line at security. I also found Copa's policy of setting the boarding time at 60 minutes for a 737 to be a little silly.

I�m flying from SFO at 12:45AM in a couple of months as well. Looking at the Priority Pass options (for those who have it) it seems like the Virgin Atlantic lounge should work from 10 to 12?

I read somewhere that it is possible to walk airside to A gates again, can anyone confirm? Also, the star alliance lounge finder indicates that I can only use the United clubs, but shouldn't I be able to access the maple leaf lougne? I'll be on the 7.59 departure with a 4 hour layover before it.

Yes, you can walk airside from A to G, but it's a rather lengthy walk all the way through B, C, D, E, and F to get there. If you're going straight from G to A, then I think it might be simpler and faster to walk landside through the International terminal departures hall to get between A and G – especially if you have TSA Precheck.

If you are booked in Copa business or are Star Gold, you do indeed have access to the AC Maple Leaf Lounge. It's in Terminal 2 D Gates. The AC MLL closes at 19h15, so it's of no use to anyone on the usual 01h30 Copa departure, but it just might work for you on the 19h59 flight. Copa starts boarding an hour before departure anyway.

Are you arriving on a UA flight? If so, you'll be arriving in D, E, F, or G. If I were you, I'd probably walk to the AC MLL (anywhere from a 2-10 minute walk), enjoy your time there, and then take the 15-20 minute walk to the Copa gate in A. Since you have the time, you could also visit the United Clubs if you're bored and want to compare. I think the food spread in the UA Clubs is actually a little bit better than the AC lounge, but everything else about the AC lounge is better (less crowded, more seating, outdoor deck, better bar service though I have not personally verified that, and showers.) Also note that Copa passengers do not get access to the UA Polaris lounge.

Thank you for the very detailed and useful answer!
Unfortunately arriving on SK, all in Y (C0 on Copa so no chance of using plus points either :-(), but *G.
Showers and bar in the AC lounge will be appreciated. Depending on the time for CBP and TSA, we might look at food options in a United Club. No pre-check as foreigns, but it'll probably be good with the walk between those flights.