First off, don't call it Frisco. :-P

Unfortunately, if you're in Y, I think you're out of luck. Both the SFO arrivals lounge and the Polaris lounge require a J ticket. You can get into a United Club, but they don't have showers.

If you have an Amex Plat card, the Amex lounge does have a single shower. In my experience, nobody ever uses it, so its easy to get into.