Attached Link: http://www.eurus.dti.ne.jp/~masasi/Service/indexE.html

Have you tried Masa's Morph Target Mirror? I have to say it rarely seems to work for me, but others report good results. You can make half a morph target, and MTM will duplicate the other half for you. Your mesh needs to be symmetrical - if you made it by mirroring in the first place, that should be ideal.