MCDM
The way the law reads can be very vague. If the Department you are working for gives you the o.k., you should be able to carry at all times under the badge. Like I said earlier try to get the P.D's policies, instead of trying to interpet MGL/c's. A few years ago a buddy of mine worked down the cape and the Department would not let summer appointees take thier firearms home, they had to leave them at the station. The reason being that most of the summer appointees were rooming together an things must of got out of hand at one time or another.
If you are sworn in by a P.D. whos policy states that you must leave your weapon at the station and thats a condition of hire, then you probably would not be covered under MGL'S/c. If there the ones who gave you the badge, thier the ones that can take it away.
Like I said earlier, I am not 100% sure of this, its just my opinion. One thing for sure is if the department requires you to leave it at the station, do it. Don't get jammed up by carrying off duty, if your not authorized to do so. Any summer app. looks great on a resume, but not if they have something like that to say about you and believe me, they will.