I have to admit, I had to look up where the semimembranosis/semitendinosis are located; and I'm uncertain of what you meant by RRL i(I was assuming right rear leg?).

From what I read, these muscles have to do with the movement of the hip and hock. I would stick with your vet on rehabbing this horse, and I WOULD NOT do any reining moves until he is sound.

Signature reining moves include slides, roll backs, and spins. All three put strain on the hind end - especially the hocks. Even the lowest of low reining classes still require these movements.

I hope the rehab goes well; keep us updated!

Oh, and I have not heard of this injury before (and I am in a reining barn).

I found this article, Functional specialisation of pelvic limb anatomy in horses. It has a good table on the function of the two muscles.