ent_mtlset is for assigning different shaders to different skins - but it assigns _only_ the shaders, _not_ the material.

For preventing misunderstandings, I've added a comment to the manual:

"This function does not set the entity material parameter. When the effect uses material skins or parameters, the material must be assigned to the entity. Material skins and parameters are shared by all skin effects."

http://manual.conitec.net/aent_mtlset.htm