Would it be possible for this mod to use additional function keys that you can't use in vanilla?

The keyboards with extended function keys are quite old and not that common, however some keyboards with G keys can be remapped to use them, provided the software support is there.

Vanilla supports up to F15, as macs do have these keys and F13-F15 works on windows 7 at least in MC when remapped to G keys on my corsair k95 rgb.

Java can support up to F24 but MC doesn't / wont use it. I just tested it in the KeyEventDemo on the Java oracle docs and they do work. https://docs.oracle.com/javase/tutorial/uiswing/events/keylistener.html

This might be outside the scope of keyboard modifiers, but it would be a rather useful addition.

Plus with some hackery you may be able to emulate the extra fuinction keys as "modifiers" i.e. shift-F12 produces F24 for example, this may get around needing explicit mod support.

You can do SHIFT, CTRL, & ALT. Once Mod devs implement our api.

Cause waiting game :P