The issue is not wether its realistic or not to do it, the question is how possible and optimized it is to implement that. Because if you use first person cover at a vehicle and it would drive away all sorts of issues and bugs could happen.
And tbh if the engine is running in neutral i don't think that the entire tank would shake around. Those things are heavy as **** and they got suspension as well.
Being realistic or not, does indeed matter.... if it's not realistic, then what's the point in wasting further time in trying to make it work?
Even if it is in neutral, if the engine is running, it doesn't matter how bouncy your suspension is or not.... the entire vehicle vibrates when the engine is running.
Go and put your hands on a running truck, a tractor, a bulldozer, a bus or something else that's big and perhaps has a diesel engine.... that is not moving but has the engine running and tell me you don't notice the vibrations?
Now try and picture yourself trying to aim your rifle on this vibrating surface and tell me you'd get an accurate shot off compared to balancing through a window or a hole in a brick wall.... or even just balancing on your knee or prone.
I actually tried this a number of years ago and depending on the distance you are from the engine or how much kick/power the engine has to it, the vibrations directly travel from the vehicle to your rifle and your sights as well.... thus making aiming with iron sights pretty difficult compared to other stationary objects. Being heavy or having suspension doesn't make a difference except when the tank is moving..... at which time you wouldn't be balancing on the tank in the first place.
Transport trucks have pretty good suspension as well as tires, not tracks.... yet put your hand on the hood or the side of the truck and tell me you don't feel any sort of vibration and then tell me that vibration wouldn't have an effect on your aiming.
In regards to coding it... I doubt it'd be anymore difficult then what happens when you walk away from a stationary object that you were using for balance.... the only difference is that it's reversed, where the object of balance is moving while you're stationary...... just remove the balance icon and abilities just as you would if you stood up or walked away.
If proximity to balance area is such n such.... then activate code for stabilization. If proximity is too great, then deactivate code.