I will be interested in hearing hear how the Trex holds up. It is a composition of recycled plastic and wood fiber (sawdust?) I have worked with the stuff and am not certain it will stand up to the rigors that a grouser pad demands. I say give it a go. You won't know if you don't try it!  Wink 
I always figured I would get some used payloader tires and cut them up with a sawzall. You might try used over-the-road truck tires too, but I am not sure they would be thick enough to stand above the grousers on the track. Check with a local tire dealer near you.The local tire dealer here handles a lot of heavy tires and always has a stack of old used stuff and I am sure they would be delighted to give them away. 
Just my thoughts, 
Cal