Retreading tires has been around a long long time. What they do is grind the remaining tread down and then add a layer of soft rubber. They then put it into a mold where it is heated to a pretty high temp. This makes it harder and adheres it to the tire.
Big trucks(truck tractors and trailors, semi's) do this often on rear axles and trailors. If you have ever been going down the highway and see a big stripof tire rubber, that's a retread, or "gator" as the truckers call them. These tires do not like heat, this can cause them to seperate. I am not so sure that they would like low air pressure either, and we all do that on the trails.
ON another note, Way back in the stone age when I was a teen, I couldn't afford new tires and regularly bought recaped "snow and mud tires" for my 48 willys. I thought they were awesome, of course at that age I also thought that I was 10' tall and bullet proof too.
I think I would use them on a DD rather than a trial rig. I have bought recapped tires with more aggressive tread and they did not ballance very good at all. So,... hope all this helps out a little.