Yes, thermal imaging could work well, I'm for it and like it for equine use. Most interior hoof abcesses are pretty painful on their own as first sign, however (think bamboo stick under your fingernail, or blood blister under your nail).
As far as the yearlings, remember that leg paints, taking bandages off, alcohol, water on the leg, ShowSheen, etc. can affect the image you see.
But interesting, what you say about what horses with hot images are turning up working, and who is not?
I know Chuck has a thermal imager, don't know if he has put it on SWLY.