Here's an example of a shirt I did with an open morph, in the modeling program in wireframe mode.  The front of the shirt is modeled in separate pieces, even though it appears closed.  It needs to be modeled this way to create the morph to open it.

I understand the concept, what I'm asking is the overall workflow for building the .cr2 from those changes: I assume you export the mesh in a closed form first as the base .obj for the clothing, then model the top button open and re-export as the first morph, but how do you export that first morph's changes to a corresponding targetGeom section in the clothing's .cr2? 

No, that's not how you do it........ The original model is rigged.

Then, you export THAT object to the modeler and create the morph

Then, in Poser, load the original shirt, Figure>Load Full Body Morph, give it the name of the object file with the morph, name the morph

Then, dial the morph on the parameters tab (which is named in the step above) to test it, zero it out and re-save the CR2.  It will contain your morph and appropriately named dial.

(I use Full Body Morph in the example above because my shirt morph example spans the neck and chest groups, therefore must be a FBM.)