I suggest that you can use the lock system to lock horses, so no one can get on them but you. You can also be able to cmodify it. My horses get stolen a lot and it would be very handy! I really hope you will be able to make this happen!
I believe there is a horse locking plugin buried deep in the bowels and ghettos of Bukkit. I don't know if it will work with essentials but if it can be done then +1
This plugin is closed source. ECC's policy is to allow only open source plugins to prevent from malicious code and to allow other developers to update and append to it.
I would think that LWC would be a much better addon for this. as lwc is currently on the server, and is reputed.
I did lock the door but someone broke in the side, I banned them from the stables now and the stables is closed until I get my horses back!