Gunter, here is another set of posts on unwrapping cones/frustums.
The script that I did was called ConeUnwrap, but I have it in customui as UnwrapCone...

http://moi3d.com/forum/index.php?webtag=MOI&msg=6175.1

- Brian

Script for unwrapping curve on a cone/frustum is possible, but I think Flow can do it.