I have 3 MA-neutered AR's here.
I bought 10 30 round pre-bans. Paid more than I would have liked to, but they came in a freaking bag with the date on them. I left one in the dated bag, sealed it in a foodsaver bag with the date visible, and keep it in my range bag. If I am 'asked by the man', I have relative proof that the mags are pre-ban. Would it stand up in court? I don't know, the courts suck. At least it is something that would stand up to relative scrutiny from LE as having made an effort to both stay within the bounds of the law and to be able to 'prove' that an effort was made to stay within the bounds of the law. My hope is that a 'reasonable' LEO would let them pass.
While shooting on our land or at my club has ever asked about them. They're usually too entertained by watching my kids shoot them.
In NH, I have as many PMAGs as I would carry in 30 & 40 round sizes for each AR I have.