If you have a propietary problem with the pz3 format why dont you just write a plugin and work out a contract with curiouslabs?
I think, because poser can Import/export in other standard format accepted in Zbrush… ( like dxf or obj)
Thats not what this is about, its about making it easy to import a whole poser figure and model it how one would like then save it again as a pz3 with all its dials and morphs for each body part still attached, understand?
Start with this tutorial by Aurick on ZBrush as a Poser enhancement, and then I would suggest some further reading in the Quicklinks, where this is from…
I don’t use Poser, but there are lots of references in the Quicklinks that would answer a lot of these questions you have… 
http://www.pixolator.com/zbc-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=001530
Pixologic won’t need to write a plugin.
If Poser supports displacement maps, with the next version of ZBrush all you will need to do is import your Poser model in obj format, remodel it to add detail and then export the detail as a displacement map to use in Poser.
In fact, you won’t be able to do anything else because Poser will not be able to support the millions of polygons that ZBrush does. Unless you want to go to the Poser forum and ask them to write a plugin and sign a contract to support the zbr file format?
They at Curiouslabs would probably be willing to do a mutual contract for the zbr file format as well, and yes poser does support displacement mapping, how much easier life would be with a plugin for poser, I could not possibly create my scenes without poser.