Hey...
In 3.1, they changed the motorcycle and mammoths from vehicles (with a healthbar) into more standard mounts.
This has me thinking you (or if not, then one of your passengers) may be able to use "preserved holly" to change the mount into a reindeer, which is NOT a multipassenger mount.
I think this could, at the very least, have some buggy effects, either graphically buggy or perhaps allowing you to "ejector seat" passengers for some mischief. (It might be really interesting if it "ejector seats" vendors from the mammoth and breaks their script, creating permanent vendors who won't despawn until a server reset. This would allow you to drop off vendors for friends in weird places and then go on to do your business elsewhere.)
But since I took a long break from WoW and don't have a multipassenger mount, I can't test this.