This may help:-

For earlier versions of Poser:

• Remove all texture maps.
• Remove all transparency maps
• Set all diffuse materials to white.
• Set all specular colours to black.
• Turn on depth cueing
• Use single infinite light pointing directly at the face.
• Adjust camera to give frontal view.
• Render -- Antialias document.
• File -- export image.
• In graphics application invert image.

See http://www.philc.net/tutorial8.php for more details.

In Poser Pro 2010 you have some extra render settings one of which is "Z Depth" which will get you what you want in one hit.

Your approach is however a valid solution.