I'm working on a new WYSIWYG tool for editing square notation. It was born as a side project of supporting neume display in Cécile, about which I talked in another thread.

The rendering engine supports the most of gabc syntax (let me know of any incompatibilities) while the UI so far handles most commonly used stuff: clefs, neumes and bars of various shapes, episemata, morae, icti, accents, lyrics, translations, above line texts, choral signs, headings.

I'm yet to implement features to customize typography, layout, and to play and export scores.

You can try out the playground here:
Guido