Updates - As before using hideProxyInCombat = 1; in the vehicle config allows the driver and cargo-turrets to turn in and out and then using 'forceHideDriver=1' gets the operation close to optimal, so we now have a vehicle that is completely enclosed, but with hatches that soldiers in cargo can turn out of and shoot.
The use of hideProxyInCombat in the car has brought up others issues though, to begin with the driver is invisible in the external view of the vehicle, and when the cargo-turrets are turned in the sound occlusion does not seem to work, I'm still working on figuring out if the config can fix this, but have not found the right settings if they do exist.