Love it, i’ve been intending to do this for a while too.
The materials are set internally to Blender so they should be translatable to external renderers just you would make them from other Blender standard materials.
On that note, I intended to incorporate a function to have some pre-sets similar to the list you get with external renderers. e.g. plastic, metal, glass, etc… (even if just taken from: http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro/Every_Material_Known_to_Man )
Preview, as said pre-make and only load and display one at a time to save memory and resources etc… (but have feature to not bother (i’d just remember))