Hmm, usually, standby on a connection scenarios would be if you want to take an earlier flight or change terminals (for example you arrive in SFO then your next flight leaves at OAK but you want to stay at SFO so you standby/ or you arrive in SFO continuing to DCA but your plans changed and you want to go to PHL so you standby) and my experience has been positive (lucky as well) so far. Always, a decent availability has to be the case. An agent once told me that it is standard for standby passengers to wait at the gate and that's where a boarding pass will be issued. But the again there are always exceptions. Red Carpet Club agents are almost always empowered to do many things and I've had agents issued me a boarding pass instantly, by passing the wait at the gate. So it depends on the agent (just be nice when you request).

In your case, your gf would have to standby for 2 flights if you don't want to pay the change fee, SAN-SFO then SFO to IAD, which would be very hard to do not to mention the hassle of not knowing. Trust me, you're gf would appreciate it not having the hassle .