Ok, I don't know if this has been suggested (but I did search the forum) or build by anyone, but:
How about a HMG with a loader position? Could just use the standard 'crouch' anim & relative position (don't know how crew positions in addons work)
Perhaps with a loader the reload time for a HMG could be zero, or the loader could run back and forth between an ammo box set near the MG?
Ok, this is sounding like it needs scripting as well, but it's just a suggestion.