After much experimentation today, I found that the only way to make it work for Poser is to

1. first export the figure base mesh as an obj file with As Morph Target, Include body part names in polygon groups, and Include figure names in polygon groups as the only options checked and then

2. import that as a tool into Zbrush. I mask out everything except the head mesh, taking care to also mask out the eyelashes and the inner mouth. I then add the target mesh as another subtool using the Subtool Master Zplugin, scale it as close as possible so that its head is more or less the same as the poser figure's head and align it as closely as possible with the eyes as a reference point.

3. Fire up ZWrap in the plugin, follow the instructions with regards to assigning corresponding points on the two meshes and then start wrapping.

4. Depending on how you mask the area between the head and neck, you will get varying degrees of separation between the two areas. You will have to then try to approximate their edges as best as you can using Zbrush's topological move brush and touch up some areas such as the eyelids.

5. Export the Zwrapped mesh as an obj file and import that into Poser with everything checked off.

6. Use the Grouping tool to select the head and neck and create props out of those areas.

7. Export the head and neck props as separate obj files and check only Morph targets.

8. Bring in the Poser figure that you want to receive the morphs. Select the head and load the head morph file using the Properties tab button. The new morph will be in the Propertes tab of the head at the bottom. Do the same for the neck.