here a nickel..

on the dress, make the vertices at the entire edge of the chest part constrained, then go ahead and run the breast simulation.  make sure everything is exactly at zero, and only the parts affected grow.   then export only the chest part as the morph target.  if there is a gap in the parts from here, it will be because the simulation also moved the edges. 

to fix this, constrain the edges of the other groups, and grow those parts too.  them export those as morph targets.  you would need to load the morphs for the chest, shoulder, collar, abdomen possibly.  but maybe these parts won't change.  loading as a morph target to the finished dress should pull the edge verts though, so unless the change is dramatic in the edges, that part probably won't be needed.  also i think you are wise to constrain the back and stuff, but you should leave a little room to stretch.

as far as i know you can't export just a part of the obj, but must export the entire obj (body part in a figure) otherwise you will have the verts becoming renumbered.  i'm sure someone will correct if i am wrong on this but i don't think so.

once you have the morphed go the the frame you like the morph at and hit file | export wavefront obj, then in the box that opens up uncheck everything except for "as a morph target".  to get the morph back into the chest area of the object, select that area and go to the parameter dials. (window | parameter dials ...in case you haven't been using them) and hit the properties tab, then add morph target.

just make sure in making the morph that the figure is in zero pose, inverse kinematics off.  go to the parameter dials, make sure that hip and body have all zeros in the x y z areas.  otherwise things will definitely get weird.

good luck i will keep an eye here for updates