Given that a concealable holster doesn't do anything but add +1 to concealment, I could see a "holster" for a machine gun (or great sword) that makes it easier to hide in some way or another, maybe by breaking up the profile, or allowing it to be strapped to your back. Sure, it's still not really concealable, but it's slightly less visible than if it was slung over your shoulder for example.
Same goes for a quick-draw holster. For something like a LMG, it would probably take the form of a harness or the like that makes it easier to unsling and ready quickly. For an axe or sword, some sort of breakaway or quick release sheath would make sense.
I've seen tool belt hammer holsters for construction that use spring clips to allow quick drawing and holstering, and the same technology would work beautifully for a battleaxe. In fact, I think if I ever bought one for a character I would probably get a Craftsman brand, because they have a lifetime warranty.