Grounding/making new ground to the chassis I would not consider hacking up the car. Where I worked, everything we did, had to be of professional quality and also be able to revert the car back in case the owners decided to sell the car. Making grounds and removing them later was acceptable practice. The seat belt mount option would leave little trace other than some scuffed up paint under the seats. What I'm trying to say, if you take this out somewhere, they're going to do the same thing if not worse.
Yes, for tracking the car, you'd have to have an external battery kill switch outside of the vehicle if you moved the battery. Not worth it IMO if this is a daily driver that you don't track often or if you plan on reselling the car.
"We live for a good time, not a long time."
RIP Colin McRae