This is a little older thread and not sure if the problem has been resolved or if your horse is just a messy eater but I thought I would give you some things to think about if you are looking to get your horses teeth floated.
Like PaintHorseMares said, it may be illegal, depending on the state. What I found this to really mean, again depending on the state, is that it is illegal for anyone other than a vet to administer drugs/sedatives to the horse, not neccesarily illegal to practice dentistry.
In my opinion, the good ones whether it be a vet or dentist can get by a horse without using the sedatives. They take their time, introduce the tools to the horse and give them frequent breaks from the speculum and procedure. Even our snorty horses do not need to be sedated to have their teeth done. Not against sedating a horse, but it seems to me the ones that are willing to take their time, want to do a good job, don't get in a hurry and don't worry about sedating to get through it. My two favorite dentists have both said the same thing, that if done right a horse actually learns to likes it and realizes that it provides relief. I don't know that for a fact just repeating.
Second, I would evaluate if a power float is the right way to go, especially on a 17 year old horse. Yes, the power float is faster but it doesn't take much to make a mistake. For example, take too much down and make it nearly impossible for your horse to chew his properly. I have known of a few occasions of this happening and the owners feeding a soft feed for a year plus on older horses before the horses' could eat properly again. A horse's teeth grow constantly, an older horse's teeth do not grow as fast as a young horse and it would take longer for him to recover from a mistake.
I won't say a vet or a dentist is the better way to go as I have seen bad jobs from both. My suggestion would be to do some research on the vet or dentist you intend to use, word of mouth goes a long way.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.