You can do so by defining a custom keyboard layout. This is quite convenient with Ukelele https://software.sil.org/ukelele/ : you can take any existing layout (e.g. the one that is currently active) and then change all key mappings, including which keys are dead keys etc. I used this to create a modified EN-US keymap where Alt-A/O/U produces the corresponding umlaut versions directly, and Alt-S produces German ß (so that when I have to write German text I can do so quickly and conveniently)

Ukulele is great! It’s also handy for disabling the default language independent Alt-letter sequences that generate umlauted characters.

This way you can have Emacs style meta key cursor movement bindings in macOS text widgets. For me M-f and M-b are indispensable.

There used to be a way to define your own keyboard decoding, but I don't know if you still can, nor how flexible it is. I'm guessing it won't be Turing complete.

I'm not sure if it's the method you're referring to, but I believe custom keybinding dicts still work. For those interested, this should tell you all you need to know: http://web.archive.org/web/20210130012801/http://www.hcs.har...

Yeah, I'm using this for λ and ∀:

    /* -*- coding: utf-8 -*- */
    {
    "~/" = ("insertText:", "λ");
    "~a" = ("insertText:", "∀ ");
    }

It looks like you can actually send arbitrary objectiveC messages to the first responder with this, but I haven't played with it much.

(I'm also now using the `fn` key to toggle between greek and US.)