It is possible to access hwp inventories and you can even assign them items that won't be stuck by creating the item and not having the "fixedWeapon: true" property, but listing the item in the builtInWeapons of the unit.
The problem then becomes that anybody can get the weapon too.. If it's recoverable, you will recover it but a new one will be created every mission. So that's bad for medikits (as it creates an infinite supply).. For the XAE attack dogs, I had a motion detector type item created for dogs, worth nothing, not recoverable, different TU cost, requires mind probe research and it was always put in the backpack.. You can put it in the dog's hand, but you can also make a soldier use it which is stupid. Kind of have to trust your players for things like this.
For an actual tank, you could make a different medikit that's not recoverable, built-in but not fixed and weighs a ton or something so it's not useful for soldiers.. Although having an inventory for the tank would open up the exploit of using the tank to throw stuff (especially if it has high strength to be able to carry the huge medikit..). So you can't make it perfect, but you can make it if you trust your player to play the game instead of exploiting it.
Final comment: I think you can actually use equipment (medikits, motion scanners, mind probes, etc.) on HWP in Yankes' OpenXCom Extended.
