The current bundled .obj exporter script in Blender doesnt work very well with Poser. Try this one.

 http://www.zoo-logique.org/3D.Blender/scripts_python/obj_io_modif228.py

 Now simply seperate the different parts (lower arm, upper arm, ect.) as different objects and name em.This way you can still have different materials as well. Blender STILL messes with the names in the .obj file, tho.("upper arm_characher", ect...). Here is a link to the thread , with some other info.(Please disregard my posts in the attatched thread because I am an idiot, lol). This script will also allow you to import and export morph targets with some success.

http://blenderartists.org/forum/showthread.php?t=73083&highlight=poser+obj