ctrl-d is already taken by “delete forward”.
You can see all the keybindings here: https://monome.org/docs/modular/teletype/manual/#keys
I would go with alt-something as that is more in keeping with the other bindings (ctrl is mainly used for text editing). It would also be nice, but not essential if it didn’t clash with an existing ctrl binding, but I’m unsure what would be best, alt-l or alt-q?
edit: the ~ key is free at the moment. Or is that “too soon™”.
Might also be nice to coordinate with @scanner_darkly and his grid display for the keystroke.