If I wasn't on vacation I would dig out the case where the SJC defined what dangerous weapons are and proceeded to say that just about anything was dangerous if used that way and that a jury could determine this, but things intended to do harm are per se dangerous. Maybe someone else can dig it up, but it's the one that defined shod foot as dangerous IIRC.
That's the answer you seek. It's not defined in law. That would make the state bound by what the law said and not flexible to make it up as they go.
Yes, I get your point that the PO could still charge you with the catch all of "carrying a DW".
But now that it is not classified as ammo, it would be no different then carrying a can of bee spray...statutorily. And unlike a shod foot, blackjack or knife (which could kill very easily) A spray from pepper spray would be a hard sell to a jury that it was meant to (or could) be as lethal. IMHO
P.S Have a nice vacation