I definitely remember making that feature, I think I even did it on stream (probably before I advertised it here), and I tested it there to be working.
That's why
has changed to
You should be able to just add extra keycodes to this list and it should work. Only on keyboard key definitions though. I think NohBoard might even crash if you try this on any other key definition.
Edit: Tested it on v0.3.1, and it indeed works as intended. For right alt, you should use keycode 165 though, and 69 for E. Put those both in the euro key you made, it obviously doesn't have a unique keycode.
See the attached file for an example.