Marsha - I would think adding boots would put more bulk in the drag area and cause you to lose your investment in your boots. I could be wrong...
Why not ask your farrier about slightly squaring off the rear toes. I have had this done on both toe draggers and ponies with stifle issues and it's helped with both.
OP - I, too, drive ponies of various sizes and work styles on all surfaces - barefoot. I've not found that we've driven enough to cause a lot of wear, in fact my working ponies have to be trimmed more often as they grow more with the extra concussion. They get wonderful, thick soles... We always have more issues with the "pasture pets" that aren't being worked then the ones that are being worked on all kinds of surfaces. I do not know how we'd handle steep paved/concrete/asphalt roads, though, since we don't have any. If we get snow, it usually becomes ice and we don't work or drive ponies on that, LOL.