When I checked out some of their work in progress videos on their site, I saw that they actually do have a muscular system in place for the upper body, so I guess it's a matter of time before they implement it for the lower body as well.
Tigersan, another technique for getting realistic muscle deformations, that may be more suitable for your workflow, is to use animated, blended displacement maps. The basic idea is that you sculpt the muscles to the right shape for various extreme poses and the resulting displacement maps then automatically blended together when you put the model in an arbitraty pose. It's a bit like joint-controlled morph targets, only better.
A user called Big Guns has demoed the technique over at this Zbrush Central
thread for his amazingly realistic Arnold Schwarzenegger model, which was completed over a period of 3 years I might add. Note that the final rendered images of the model are at the end of the thread. He said he is going to post a "making of" article for this model and, even though it's a male bodybuilder, I'm sure there's still a lot that we could all learn from studying his techniques.
