So far so good.  One thing I should have clarified -- the smile is already an existing morph on the figure -- but so far the steps I've taken seem to be the way to go:

I exported the smiling head as an object (posed) into ZBrush with GoZ.  Then in ZBrush I created a new layer, set it to "1" and turned on record mode.  Then back in Poser I zeroed the face and exported as an object into ZBrush with GoZ (note: I haven't yet completed the round trip, so this makes 2 exports to ZBrush without re-importing to Poser).  

Since I had record mode on, the zero-ed head immediately replaced the smiling head as a new layer and voila, I have a smile morph slider in Zbrush.  Now I'll sculpt the wrinkles, etc. as a second layer, dial down the smile, export a new displacement map that should fit! 

Fingers crossed. :)