I didn't see any, either. Regarding the subject of retreads on steer axles... I've never done it, and don't plan to do it on our work trucks. I don't think it's legal if you surpass a certain weight rating per tire, which most of my trucks do. I would, however, consider it for my personal pickup, if I had 19. 5s.
Why does DOT not want them on school bus steers? Probably liability, like most things. If a steer blew, and it happened to be a retread, it would be blamed on "that damn retread". I'm sure you've read some of the studies showing how "road gators" are perceived. Almost everybody assumes they are retreads because that's what they've been told. However, the majority of gators still have the steel belts attached... which implies it's the casing that blew out (since retreads have no steel belts). And when the inspectors could find a "mostly intact" casing, they found sidewall heat damage, caused by under inflation.
Back around September/October when I bought my regroover and siper, I started by practicing on old truck tires I had and gators I found on the road. I grabbed 12 samples, and every sample I grabbed off the shoulder of the road had the steel belts attached to the retreads or the original treads.
I'm sure when retreads came out, they had issues, but they don't seem to now. According to Bandag, the cushion layer (between belts and retread) is the strongest part of the whole tire. Personally, I believe it... and I'm skeptical about everything unless I have good reason to believe otherwise.
Same goes for regrooving. Liability. Most tire places don't regroove tires, because if anything happened to that tire, it would fall back on the last person to touch it. They would get sued, and their insurance company either would drop them, or raise their premiums to the point that they would stop offering the service.