I don't think this is possible with standard lite-c functions. You can however use 'ent_getmesh' to grab a DirectX mesh of a specific block and then I guess you can use any of the dx functions to map a texture onto it, eg, SetTexture, SetTextureStageState, etc.

This requires knowledge of DirectX though.

Maybe conitec should add 'ent_for_block' / 'ent_to_block' functions?