Thank you for your hints!

@crumply
It's a pity, but I can't use bones functions.
I made this model with Milkshape and imported it in MED.
Unfortunately the mesh at some bones animations is deformed.
Therefore I converted the bones animations in vertex animations.
The vertex animations work fine for me.
Perhaps I could use vertex functions like vec_for_vertex to mark the points of the group and hide the mesh, but it's complecated.
It would be much easier the hide a group over its group number, but it seems that Lite-C has no function to allow this.

@DJBMASTER
It's a nice idea the use transparent skins and make them invisible.
I'll try it out.