One item not used in the game that can be activated is the Flak 88. Unfortunately it has the wrong sound from a MP40. There is a self-propelled Flak 88 in the Tiger Tomb mod. You can also let a mounted weapon shoot bazooka or Panzerschreck ammo.
If you give a mounted weapon the item id of the flamethrower, it’s just like a Flak 38, so it doesn’t exist in the game. There’s also a model of the Vickers K gun in the game but not as a usable weapon.