Hey Martin,
You are right that something extra is needed to make any custom synth accept things like note names.
(For a little more context, a similar question has been raised previously at Trouble adding new Synthdefs for example)
However, off the top of my head, as far as I remember it would currently be necessary to add a new section into the synthinfo.rb file that contains the metadata for your synth, and within this, you would override a particular function which ‘munges’ the opts before they are sent to SuperCollider, where you would in turn call the function that converts the note names to midi numbers. A bit vague as instructions go, but I’ll add a bit more detail tomorrow if desired and someone else doesn’t chime in first! (It’s late here and I’m off to work in the morning after 3 week’s break) 