I don't see an issue with it. THey already know how to change leads, break & go fast, they just have to get the turns down.
The lady across the road has had a couple that she retrained for barrels & she loved them. There was also a gelding I was looking into years ago that had been raced, then used for some jumping before being lightly trained for barrels. He was 16.3 hh & got 18 seconds on his very first time doing the pattern ;)
I know a smaller, shorterbacked QH is more popular because they tend to turn quicker, but big TB's make up for it with their speed & long strides!
I think it's so funny how good TB's are at barrels. They're generally such big horses, most barrel racers are smaller. But I can see them being pretty good with their long legs & OTTB's already have the balance, changes and speed. So I say go for it.
It's safe to run barrels on a horse that is over 16hh! All of my barrel horses have been over 16hh, and made good times, 16 seconds on a big pattern. My current mare that I'm training is 16.3 hh, and turns like a demon. If you teach a big horse flexibility, they can make good time going around the barrels too! I used to have a gelding that was 16.2hh and 8' long from nose to tail, and he could rim a barrel all the way around without knocking it over! Big horses can and do compete well in barrel racing!
Actually infact that's exactly what I ride if you look at my picture that is my tb. And I race her as well. She is flippin fast. I am thinkin about breeding her and putting her colt in racing then moving him straight to barrels. But my horse is my pole horse we both love those better then barrels but she can still do barrels