way over my head but instancing i far more than just modelling in a mirrored`way, if you look at the options in the 'duplicate' action you will see 'copy '(i think its called) and 'instancing'. so you make duplcate of the geometry and when you change one the other changes wiith it, or thats my understanding, cant check, puter down. ( i am sure you already know this, but do not underestimate instancing, especially with particles)
looks like you will save time just going in there and doing the geometry the old fashioned way, by hand, while you wait for an answer that may or may not come ( I meam mirroring a blend shape and duplicatng it at the same time doesn't sound poss to me, but then again, maya can do anything, nearly)
some great work there!
i think this new post of yours will definately help those who can answer this to understand your needs.
OK wheres the big guns?? lads??