Heck, paying members can be denied if the club is full
SFO appears to be somewhat sticky about rules, although all my trips through the UA int'l lounge have been pleasant. I would suspect, if you're on UA tickets, you'll have better luck there than at the SQ lounge.
The thread, linked below, although with an older start date, should give a general impression of how things have gone:
http://flyertalk.com/forum/showthread.php?t=334512
I think this is one instance where RCC membership would've paid off, getting MIL in.....
Shoulda upgraded MIL to C....
Pat