From my experience, most of the PAs on Bway shows are Equity. Basically, you PA the show as sort of entry-level, building a relationship with either the PSM in hopes of getting an ASM position on one of their future shows or with the producers in the hopes of getting an ASM position on the tour. The logic of it being that you are not working solely for the stage management department; you are working as an assistant for the entire production and therefore any department can use you and therefore not under AEA jurisdiction... It's a fine line...
I myself have done it. How did I feel about making $400 while the everyone else was making production contract (and myself, coming off a show where I was making nearly Prod Contract rates)? Just fine, because I'd have done it for free... I learned an incredible amount getting the opportunity to watch the creation of a Bway show, getting to see the way another PSM worked, see another set of paperwork and I was able to make some contacts. There were times that I was working my butt off, but also a lot of time where I was able to just observe and learn...