Thanks for the comments and answers so far.
I did a bit more reading, and it seems that writing a lexer for textadept is really quite straightforward, since textadept uses Lua for its configuration. Textadept 11.4 API Documentation
So, I got up and made a pot of coffee this morning, and now I’m writing a Julia lexer for textadept. I will share it here once it is in a state I am happy with.
Cheers, Timothy