Rather then a field hospital how about an unarmed BH that works as a mobile "health station". Your squad just got done assaulting a position and you've suffered heavy casualties, your medic is dead and your team isn't in the greatest health (low stamina aswell). You call in the Medvac Chopper which can load your guys onto it and heal them aswell as resuply the field dressings, and restore stamina. Since the chopper is unarmed and a Medic chopper, having an enemy shoot it down would cost the team that shot it down tickets.
Pros:
Would provide mass healing both quickly and safely
Would restore stamina
Would restore field dressings
Can transport the squad
Cons:
Unarmed
Could be abused as a "hop in and heal, hop out" vehicle
Could be hard to distinguish it from an armed troop transport BH
Solution for 2nd Con lol: Make it so that the healing doesn't start immediately, but rather happen gradually, that way if you hop into the BH and sit there, you are a giant target for enemy small arms
Counter Argument for "Solution for 2nd Con lol" Since you would lose tickets for killing a Medical chopper I guess they would be invincible....

Ideas to make this a better idea lol.