To be honest, I was thinking about a script like this and I really started writing one as soon as I learned scripting... It was several months ago and as you can see, there's no head plug-in now because I left this idea as soon as I started.
In fact it's not very difficult to write a script like this, it's just A LOT of repetitive work and the script file would have thousands of lines. I just thought it's not worth the work, everybody can create his own head (well, everybody should try it) or you can find a free head model and tweak the shape...
Another problem is that Anim8or only allows 10 parameters for shape plug-ins and that might not be enough for a script like this. As you said, one of the parameters will be "size", another one should be "level of detail"... And you have only 8 numbers to describe all possible shapes of a face! I think this wouldn't work...
Ok, I never really left this idea, but if I ever start working on this script, don't expect it to be in the next few months...
