Is it possible to model in Apprentice HD, export to ZBrush for touch-up, and back to Apprentice HD for rig, animation and render?

Sure, I can't remember what formats zbrush will accept but I would assume .obj.
So, model in houdini and at the end of the sop node network RMB on the node you want to save geometry from as a .obj file.

Bring that into Zbrush, do your thing there, save that out to .obj and bring that file back into houdini with a File sop.

Also check the normal map examples on the Exchange [sidefx.com]:
http://www.sidefx.com/exchange/info.php?fileid=562&versionid=562 [sidefx.com]

Thanks guys!

That's superb!