Well, the reason I don't use GDB is because it's not supported for the make-prg'd Zehn files (is that still true?)

Only on 3.1, if you use 3.6, you can debug all Zehn executables, even in PRG containers.

and I use 3.1 for testing stuff (because it's faster, especially with file transfers). If Zehn support is backported to 3.1, I'll probably use it.

It's not that hard, the easiest way is probably to make the ndless_resources from 3.6 backwards compatible for 3.1. Just a few lines, I'd imagine.

Despite my limited spare time, I had to try it and it was just a bit of copy & you can guess the rest
Tested on emulator CX CAS 3.1.0.392 and 3.6.0.something, not on other models so it may even crash on startup which would be annoying.
The ndless_installer from 3.1 still has to be used. Tell me if it works, although some weirdness with lua and similiar things are expected.
You have to rename the file to ndless_resources.tns of course. Then reboot and you're good to go.
It doesn't support ndless 3.1 uninstallation!

I thought it already was?