Same experience here. Just got back from a six-day driving trip where my wife and I used McDonalds restrooms without issue multiple times. Some of them were even quite nice, as such things go. But I've also found myself in dire straits in cities, with nothing truly public and the McDonalds/Starbucks all locked down. The most dangerous place I've been in the last several years was an MBTA restroom (I was desperate). So much for "walkable cities" I guess, but good luck finding a proponent who will engage with that aspect.
You're not really that far from either Faneuil Hall/Quincy Market or North Station there. Can't speak to how easy it is to find one in any of the local hotels. Of course, in the latter case it helps to look the part and not look too clueless.
> Of course, in the latter case it helps to look the part and not look too clueless.
This is not just applicable to using the restroom as a non-paying customer. You can pretty much go anywhere if you just don't look guilty of doing something your not meant to be doing. They also say carrying a clipboard and looking in a hurry increases your "getting away" with it, whatever "it" may be.