Success!

Special thanks to goast666 on the ZBrush Central forums for helping out with the import workaround.

The Problem: ...was happening not on export from ZBrush or even import on ZBrush, but importing directly into an existing subtool level to update the UV's with UV's laid out in an external app (in this case Maya).

The Solution:
1. "Store" a "Morph Target" on the subtool layer before you import the new OBJ.
2. Import the OBJ. At this point the scale and position will be totally off.
3. Hit the "Switch" button in the morph target UI to snap the newly imported OBJ into the correct position and scale.
4. Then "Delete MT" (delete the morph target) for a quick clean up.

Worked like a charm and kept all the new UV's intact.

Thanks again goast.