I was pondering the concept of armor/horse taking up an equipment slot, but this is definitely more fleshed out. It will be interesting if it's possible. 2 sounds more difficult.
1. I think that only the 6+ difficulty horses should have this alternative mode. For example, Desert Horse With Stirrups would beat out the standard arabian in almost every practical way. A % increase would favor those lighter horses quite a bit, how about a flat +10k?
2. Because this would be usable on every horse and replaces boots, it would need to be very expensive. 20k.
1 can include all horses and 2 be reduced to 15k if the desert horse gets 41 speed/45 maneuver instead. Also, I am assuming that this would give +2 maneuver.