Ah. Unfortunately, I don’t use G services, not even for official news from Purism 
At least it has been possible. What gargron seems to have done, in the commit I linked, is to make the server endpoint a dead end. Whether that is the endpoint where other servers connect or where apps connect I don’t know. (Mastodon has different protocols for s2s and c2s, IIRC.) Reading the code is a possibility, but…
Wonder how other Mastodon instances react, when Purism Social refuses PMs. If they were already built to handle that situation and have a means to notify the sender, or not.