Dave I'm not sure we're saying different things.. as you still say it's technically possible, but not a good idea. agreed.
But if you were transmit on 70cm with a 2m antenna to a truck that's is 50' behind you, I'm not sure he wouldn't hear you. We can try with one of your radios..
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 06 TW200, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50