Thank you Wolfgang!

Your example works as intended.

I really get lost in the font definition files, as it is difficult to
understand which variable point to what other definition.

Is there a (simple) picture where the scripts are annotated with arrows,
to know which variable points to where? All the way back to the
mtxrun --script fonts --list --all --pattern=*
reply?
With all the options available it is a steep learning curve, but i hope
to be able to understand it at some point.

Thanks for the working solution.
.Floris