Well, this is certainly making me want to go put my car in the air to check it out while I have it off the road and time to work on it.
Peter, I know what you mean about car rentals. If I had a spare of my own that was on the road, I'd drive it up and let you borrow it (since it would be a spare and it could use some paint too
I would think that if you were careful with it, you should be fine. The motor/tranny should be well enough balanced in there to not be a huge issue on open roads. Just on the bumps.
You may want to see if you can get a ratchet strap in there somewhere so you can ude that to hold the subframe up. Not sure where it would attach, but would keep it from banging into the body and the reduced movement would help save the other mounts.