Hey Mike

You may already know this but an easy way to preserve your materials if Zbrush eats them is to
load what ever changes you make on the already mat/grouped original in poser.  As long as it's just sculpting ( not sub-d or geometry) changes your good. I do this alot when making morphs with Blender ( It eats materials too Lol ). In poser just load your sculpt on the original mat/grouped version as a morph target dial it to 1.0 then export. Use the export option "Include existing groups in polygon groups". You can also use uvmapper to get groups back as well.