WMBR does not yet have PPM. They say they may get it someday, but since they are really not concerned about Arbitron ratings, it hasn't seemed worth it for them to invest in the equipment so far.
I don't know for sure, but I don't think that WZBC, WMFO, WMWM, WBRS or WRBB have it either, and I'm guessing that those stations probably feel the same way.
Under the old diary system, WMBR and WZBC used to average about a 0.1 share. The stations with signals that only covered a portion of the market such as WMFO, WMWM, WBRS, WRBB were always below 0.1. Why should these stations invest in buying and then maintaining the equipment just to see these small numbers?
As a longtime staff member of WMBR, I can say that they feel no need to attempt to maximize their ratings. They do very well with their annual on-air fundraiser, it more than covers their expenses, maintenance, and upkeep as an all-volunteer operation. Their programming is all niche and specialty shows, and the station often completely changes format every two hours. That produces a number of different "cult followings" who tune in for, and passionately support, their favorite shows, but it's not favorable to overall Arbitron ratings, which take TSL (Time Spent Listening) into account, etc... Unlike majorly "single-format" college stations that actually pursue Arbitron ratings with the same or similar programming all day (such as WERS), only a small amount of listeners are multi-genre adventurous enough to leave a station like WMBR on all day long outside of their favorite specialty shows. And, that's the way they choose to program it, to give as many MIT students and community volunteers as possible the chance to present unique and truly diverse programming, which always yields more than sufficient fundraising support for them as the modest operation that they are.
WZBC has more of their weekday schedule devoted to "block" programming than WMBR, but that programming is intentionally extremely eclectic in the alt-rock and experimental avant-garde music veins, and is also not intended to draw Arbitron ratings. Again, they seem to get by just fine, only needing to fundraise on-air one week every other year.
WGAO actually covers a good chunk of ground in the Boston outer metro southwest suburbs and south central MA, and they are largely formatted like an aspiring commercial station. Like WERS, they sound like they are concerned with pursuing Arbitron ratings, and they are showing up. I wish them luck and more power to them, but stations like WMBR, WZBC, WMFO, WBRS, etc... choose to march to a different drummer that is not necessarily mass appeal and ratings-friendly even in the non-comm realm, and in my opinion (and in those of their supportive "cult followings" of listeners), thank goodness that they do!