How much do you trust your vet's opinion? I have the utmost faith in my horse's vet, and would take his opinion over another rider/trainer's any day. Perhaps you could get a second opinion from another vet?
Remember, every break and every horse is different. Perhaps the other horses who have continued their barrel racing careers had breaks that have a higher success rate for healing. BR is an incredibly strenuous event (as I'm sure you know); that is a lot of strain on even healthy legs. If there is a weakness, you could be headed for disaster.
Honestly, if my horse had recovered from a fracture, I'd be thanking my lucky stars that he was not only still around, but sound. I don't think I would ever risk him with something like barrel racing. My own horse has spent close to a year recovering from a pulled suspensory ligament, and I doubt I will ever let him jump again. He's first and foremost my dressage horse anyway, but he did enjoy jumping a couple times a month. Right now, I'm just happy he has recovered.
Like I said, if you are really set on BR with him, I would contact another vet for a second opinion. Where in Florida is Quincy? If you are located near either West Palm or Ocala, you have access this time of year to probably some of the best sport horse vets in the country.