Thank you for sharing your experiences! That is so awesome.
I'd wondered if it would work for others similar to what I'd started to experience or not... The bad thing is that this year for us over all has been so in-consistent with working ponies at all - due to LIFE.
Another two exercises that I've used while riding, but not really yet while driving, is the "point to point" or "post to post" - you choose a spot to drive to, continue to look at it and pick a speed and your horse goes to it. When you arrive at your "post", you turn and pick a new spot and go to it. When riding, you do this on a loose or draped rein and you let your horse "make the mistake" of going the wrong way, straying off the direction or changing speeds and that is when you pick up your rein and correct him. Then release to go again (as you've already stated that you do). I believe when driving that you really don't want a draped or loose rein - maybe just loose enough to still allow them to "make the mistake" and you correct them, then "turn them loose again" - but really having somewhat light contact. What do you think? I LIKE this exercise, when riding. Works very, very well with our arabs and they learned that they didn't have to rush everywhere.
The other one I think you could do while driving is "Cruise Control". This one was hard for me when riding - think it might be almost impossible while driving -not sure. You pick the speed (tell your horse to go) but he can go anywhere he wants in the arena. your only thing is to have him go the speed you want. Even with driving - this means one rein for slowing, stopping, correcting. I just know that I have a hard time "turning them loose".
For both of these exercises, I'd want to have a fenced area to work within not an open field, but I could see eventually using pasture obstacles.