I'm not sure if this is possible, so it's a suggestion.
For the people who played Half-Life 2, I'm sure you'll be familiar with a massive crane you use in the level "Highway 17". In essence, this vehicle would be that crane. It would not be capable of driving, but moving it's magnet and turntable. It would be capable of picking up stuff that is classed as a "vehicle". It would also have to have the near-indestructibility of the Bench vehicle due to it's size. It would also (obviously) have only one mountpoint. If it couldn't be made with a full-fledged set of legs, it could have only an upper-part and no legs. It would also have to be incapable of being pushed or picked up by Spaceguy's Gravity Gun (weight).
If this is impossible, tell me and I'll lock the topic.