It's pretty crazy that the instrument approach for Cambridge isn't protected airspace, considering that reasonable numbers of heavy airliners / C130s / bizjets etc all use it on a pretty regular basis.
I know it's "legal"
to do so, but it's not really common sense. If I was a controller and I had a B747 or a Herc on 4 mile final in IMC, and then a PPL made a "courtesy" contact call just to let me know that he was transiting the overhead, I'd be annoyed too. If the big guy made a go around for whatever reason, you'd be in prime position to go straight through his intakes.