Could you potentially ignore the player body while building (just allow grabbing and interaction of parts)? You could have a visual indicator and not allow spawning of a new piece by testing if there's a collision. Perhaps there could be a toggle for a 'drive mode' or contact mode that allows you to continue building while you're on top of a vehicle/structure.
You've probably already done something like this but just figured I'd throw out some ideas. Will be checking back for the update!