Hey, yeah! I totally understand that Engineer would basically be as useless as a box of rocks. I'm actually in favor of your idea of attaching buildings on the vehicle.
From what I recall in my days of Zombie Fortress, there was a map with an elevator that let you build on it, but when you did the buildings would be permanent fixtures in the air. To fix this is easy, all that needs to be done is place a trigger that parents the building to the prop.
Surprisingly, it works fine! Only problem is the buildings can hang off the edge ( sometimes to the degree of not touching the trigger to attach it to the cart ) and teleporters do make the player's camera angle wonky when teleported to. The surprising part is the building can still be hauled and WON'T be attached to the cart anymore! Thanks Source.
All in all, quality suggestion!
EDIT: ALSO, our time zones are like 11 hours apart. (I'm -8 GMT) and it's about 10 PM (22 o'clock) the time I post this. Let's message each other and try to coordinate a test! I'm going to see if I can finish a2 in 2 hours so I can roll out a test.