Just checked out the DSHS site for TX and in 2012, there were 683 reports of rabid animals...but only 4 were horses and each were at a different corner of the state. The majority of the cases were bats, skunks, foxes, and dogs. Most of the cases were centered in East and Souteast Texas; from DFW and south from there.
1. Who can vaccinate: We, the horse owners, a number of years ago could vaccinate ourselves - which I did. As explained to me by my vet there were a couple of things that triggered legislated vet only vaccinations where I am. One, a farmer apparently vaccinated one of his cattle for rabies and it came down with the disease anyway (that raised questions from the governing vet board on proper administration of the vaccine) and Two, people started vaccinating all different kinds of animals such as racoons (while that in itself is not necessary a bad thing it presented difficulties in doing any sort of meaningful monitoring and tracking of the disease).
2. Annual vaccinating: I get my vet to come out every spring principally for dental checkup and floating if required. At that time, I also get the west nile/sleeping sickness/tetanus plus rabies done. They don't charge me for anything other than the cost of the vaccines so there is no financial advantage to doing it myself. Also, where I am after rabies is given for two years straight, you can revert to doing it every other year after that since there is apparently enough antibodies built up in the horses' systems to allow that.
3. My brush with the rabies police: Awhile back I broke up a fight between two of my cats out in the barn. I took two bad bites to my hand from the one of them (man that hurt - I thought I was going to faint from the pain; it was my fault completely by the way as I saw the danger signs but chose to ignore them). Consequently, I went to a medicentre for a tetanus shot and some antibiotics. Somehow the story got twisted between the medicentre doctor and the province (animal bites are reported) and I receive a call from a health inspector. She thinks I got bit by a strange cat that was in a park and that's of great concern to them. I give her the accurate facts that these are my cats, they are vaccinated (thank goodness for that) and it happened on my property so then it becomes just a question of calling me back in thirty days to check on my health.
Rabies vaccines are considered to be extremely effective. Animals that are bitten by rabid animals are revaccinated just as a precaution. The risk of human exposure and death makes the caution necessary. To say that the vaccines are not worth giving is incorrect.
I haven't read through every post, so forgive if this is just a repeat of what is above.
I do not know of a law that prohibits one from vaccinating their own horse w rabbies vaccine. A given state (such as NM and several others) might restrict the sale of the vaccine to vets only, though.
I do not know about all states, but in many western states public notice is given for rabbies vaccination for horses per county only during a verified outbreak (I don't know if it is "suggested" or "required"). An outbreak can consist of a single verified case, including (and ordinarily is) wild animals.
So, if there is an outbreak and you do personally administer it, you will have protected your horses from the disease, but not yourself from the law ( if there should be any question), as you will have no certification, and you will have no traceability (that will hold up) in the event the vaccine causes problems. Personally, if it weren't restricted here, I would just administer it myself.