I found DiaGen which is a Diagram Editor Generator, for creating editors for specific customised types of diagrams, which sounded promising.
It even has an example of a Syntax Diagram Editor:
Which might have been good if it wasn't such a clunky piece of shite, and a stereotypical example of a unresponsive Java GUI, and lacking in basic features (like keyboard shortcuts).
Still, if I search more for "syntax diagram editor" I might actually find a better version of what I'm looking for. :)
