Do you think its a relatively 'fresh' type of incident...does it involve both the glute and or hamstrings only? My GF's mare actually won her divisions with this type of injury - her owner/rider didn't realize it...(how, I don't know!) but it is fairly marked at a walk with a short, choppier LH stride.
A couple years later she decided to go with shockwave, after her vet suggested it could help. It did, to a degree. But I imagine there's various degrees of deficits with FM involving the hinds. Surgery was not offered; good to do more research!