Tofusan, multiple surface selection with different materials can be implemented in a manner similar to the way DAZ studio deals with different parameter values.  If the Lux render material types differ, then just display "Multiple"with no other material parameters.  When the material type has been selected, then the relevant parameters can be displayed and set.

As for libraries, you could have two buttons on the materials tab -- one to load material from an lxm or lbm2 file, and one to save a material to an lxm or lbm2 file.

Your materials handling is super as is.  The addition of a materials library and the ability to set materials for multiple surfaces would make it unbeatable.