adam.peterson44 said: "Disability and use of accessible facilities is by self-identification. If you don't feel able to use stairs in your "super-preggo" state, then that is your call, and it is fine to use the accessible washroom if you need it."
Yes. (And Jujamcyn staff are the BEST)
I'm an usher at a theater in SF with a similar situation. We have 3 single ADA restrooms in orch and a line. Working the ADA line is one of my favorite positions. As my husband needs to use it when he goes to shows, I have somewhat of a personal stake. When I mention that to the people in the queue as they wait, they usually get that "so you understand!" expression. We always announce that they are "accessible ADA restrooms." (Sometimes, if it's a big family show, we also add "family" to the description.) If people ask for more explanation, I say that it's for people who would have a problem with the stairs. Of course, announcements are followed by mentioning the 20 stalls in the large women's bathroom down the steps.
Of course, we never make people "prove" they belong in line. When asked about it one day, I replied that not all disabilities are necessarily obvious to others. A few minutes later, a patron thanked me for saying that. While I never single anyone out, every few minutes, inevitably a few people get in line, unaware of the ADA purpose. I then will make the general announcement again (but not directed to anyone in particular). Usually those who were in the line by mistake head to the big restrooms.