All good points. I don't typically put anything on my back on a trail ride, particularly something hard like a radio. Something like that can crush a vertebra, turning a nasty fall into a life-altering event, so it's pretty much going to have to stay on the belt.
I understand. I kept mine in a backpack I was wearing anyway, and off to one side. I was thrown off the ATV once, and broke my hip socket and cracked my pelvis in 3 places. The radio didn't cause the injury, but that's good thinking if you go straight off the back.
I can't put it in a saddlebag because in the event of an unscheduled dismount, I want to have it with me (assuming it will still work).
Yes, that's important. Having it attached to me (in the backpack) was extremely helpful since I was the last guy on the ride, and I needed the radio to call them and come back for me. Not having the radio with the rider is a bad idea.
I do have a BNC connector on my radio, which should help some, but I also just accept the fact that I'll probably break my radio someday. To your other above comment, I don't need very long range. I'd use a stubby if I were confident I could get 2 miles out of it.
Better than the stubby, go with the 19" long thin whips. That is what I used on a VX-170 and it performed pretty well for a hand held. It's flexible enough to not annoy the person carrying the radio, and it gives you some good range. The stiff/stubby antennas can still put a lot of strain on the antenna connector.