I dont think you can achieve muscle balance through isometric holds. At least, not in the popular definition of muscle balance. Yoga could be used to encourage muscular balance, but the process can be long and drawn out.
After reading through some summaries of Soviet strength coaches, I think that one of the best tools for muscular balance is through cross training and avoiding specializing in young athletes. Baseball, for example, encourages single side dominance due to throwing and swinging with a single side of the body. Sports like soccer and basketball use both sides of the body, and therefore can function as a re-balancing activity. I've been reading quite a bit of stuff by Scott Sonnon regarding the use of yoga and circular strength training as a means to break the various imbalances caused by sports. You might find it really interesting RIStar. Some of his stuff is "out there," but you cant argue with his results. This video shows him doing some strength work: http://www.youtube.com/watch?v=LAOFtiHRsxs