It's an interesting idea, but the only way to balance it would be to force squad-only spawning on all vehicles for everyone. Otherwise one team will have a team-wide spawn location in the field, while the other team will be missing out on that spawn location cause that vehicle would be squad-only.
I see exactly what you mean. So the idea would be that it would strictly effect the gunner seat (granted this could be problematic to code), additionally the feature would be off by default and could be reset with every vehicle spawn. Is there anything else you might add?
Edit made to clarify language
