It's inside the main part of the library, so plenty of staffers hover right nearby. Big John sez, check it out. Go in the main entrance and make a hard right once you pass the front desk. Or just ask the friendly librarian. It's a bus station with the only public restrooms on the south end of Pacific.
Restrooms are inside the main building, and security's reasonable.
In past years the men's room was nigh unusable, but Metro management have in recent years upgraded to the latest in armor-plated bathroom fixtures. And a good thing, too, because the Metro restrooms are heavily used, and get a real beating from a few of the folks who hang out at the Metro Center. Big John's not blaming bus passengers in general, who're generally considerate folks like the rest of us. But the restrooms also get heavy use from everybody who habitually hangs out on the south end of Pacific. And a certain number of these folks are low on hygiene skills or just don't care to clean up after themselves.