[ubolonton_]

In Emacs, local-function-key-map has this entry:

    C-x @ s                   event-apply-super-modifier

So to get s-g to work, you'd configure the terminal emulator to convert the Command+G key press to the C-x @ s g sequence.

    # Hex code (iTerm)
    0x18 0x40 0x73 0x67

    # kitty (without Emacs doing the integration)
    map cmd+g send_text all \x18@sg

Konsole somehow seems to have this done for the whole English alphabet, so it just works out of the box there.

For 2-modifier bindings, I guess you can add a creative entry to local-function-key-map. (If the other modifier is Shift, you can just use uppercase letters.)