Thankfully, there is no "end user license agreement" with most things. Once you buy it, it's yours to use, destroy, modify, whatnot. Just don't expect post-modifIcation warranty service.
Mag's primary interest is going to be maintaining their brand recognition and distinction.
You might be able to go commercial with modified maglights and openly state that you use genuine maglight, just don't state that you're associated with, endorsed/licensed by, or a division of Mag Instruments. I think there is at least one CPF sponsoring-manufacturer that does just this.
I'd be surprised if Mag would sell you lights or components in bulk. That alone prevents serious competition from "mods" since the pricing is never competitive.
They might also threaten legal action if a custom part you designed for your product bears more than a passing similarity to one of their own.