Curious,
The MGLs ONLY prohibit firearms on ANY school property, this includes colleges and even if the school property were rented for a private function.
Anyplace can "prohibit weapons", do a search and ask you to leave. Refusal to leave amounts to trespass and you can be arrested, BUT the mere possession of a firearm is not a crime on any private/public property (you should check fish and game, federal parks, etc. laws for property that comes under these regs/laws as I don't know about them) regarding MGLs.
Also, if you actually read the WHOLE law referenced in the short poster at the USPO, you will find that it is intentionally VERY MISLEADING! The actual law has an exemption (to the prohibition of having guns on USPO property) "for any lawful purpose" . . . e.g. if you have a LTC and going in to do normal postal business (no intent of robbery or shooting up the place) you can carry legally at the USPO.
Courthouses and some city halls now have security and won't let anyone carry any kind of weapons, cell phones (e.g. Moakley Courthouse), pagers, PDAs, etc. but it is not really LAW (MGL anyway), but policy that they can legally enforce (by using "trespassing" as the crime for refusal to comply).
LEOs are usually exempt from the above legal restrictions, but may still have to give up weapons in courthouses, etc. if their policies require it.
Also, as stated, MGL makes it illegal to carry if under the influence of alcohol, medication, drugs, etc. If found out, you'd likely lose the LTC permanently as a "not suitable person" anyway. I would suspect this to also be true if you possessed a LTC (but were NOT carrying) and were tagged for being under the influence (even if not driving).