Yeah, I know I was vague in writing that section. There is a scenario that uses that functionality, but I haven't been able to figure out how to do it. I dont think you can change animations through it so your horse idea wouldnt work.
The only way I can figure out to get a unit to appear on horseback or not would be to have 2 seperate models, one mounted and one not. Then you would need and SDK change to intercept where the art definition is pulled and redirect it to another art definition if the unit has the promotion.
Impaler could probably put something together like that in a few minutes. It actually wouldn't be hard to create a unit art style on a promotion that would cause units with that promotion to use that unit art style. So if a swordsman has the "mounted" promotion he uses the UNIT_ART_DEF_SWORDSMAN_MOUNTED model instead of the normal one. Or a wizard that has the undead promotion uses the UNIT_ART_DEF_LICH model instead of the normal one, etc.
The big change in BtS (thanks to Impalers code) is that you don't need new units for all of this, just art definitions. It makes it so much simpliar.