In the course of writing my mod (Dude! Where's my Horse?), which is specifically about locating and summoning horses that have wandered off, I've been implementing compatibility with your mod. Thankfully, as you derive from AbstractHorse, I don't actually need to do much at all.
The only two "issues" per se are:
Uncertain of how or if Alea, Asmidiske, etc, spawn naturally: when spawned with an egg these are specifically saddled & marked as tame, but are marked without owner.
The Vagabond Knight, upon his death, leaves behind a similar tamed & saddled horse which does not have an owner.
While I have a consumable item to tame horses in my mod (which is enabled/disabled via configuration), it's a simple matter for me to have it manually mark the user as the owner of an unowned but tamed horse.
Specifically as to prevent chaos in multiplayer servers, I rely on the owner IDs matching before a horse is properly summoned.
I'm not sure your intent with the specific named horses, but as it appears the Vagabond can spawn I took the liberty of including a death event handler in my proxy for your mod which:
When a player kills it, sets the player who got the killing blow as the owner of the horse
When killed by any other entity or effect, untame & unsaddle the horse and drop the saddle as an item nearby.
I noticed on the Minecraft Forum someone recently commented about finding a random tamed & saddled horse that nobody laid claim to.
It's definitely outside of the scope of my mod, so I wanted to make the suggestion that perhaps you could implement some method of handling this. If you're interested in seeing my solution, the code is currently UnicornProxy::onVagabondDeath at this location.
While I haven't actually played your mod myself, I have enjoyed playing around with all of the horses and noticing the interest effects that breeding has while I was testing the breeding effect of my "enchanted carrot" item.
Have a great day!