Hello again.
Currently it seems that transparent text rendering isn't possible in ClanLib(as of version 2.2 I'm using). I thought it should work by assigning alpha value to the color of the rendered text but it doesn't working.
I've made my own little dirty fix for my goals but it is ugly and working only for sprite fonts so I really can't suggest it as patch for ClanLib. So it would be nice if someone could implement this feature pretty and correctly.