> Sorry for the delay. I've tried implementing this, but I'm a bit at a loss
> here: If I understand the GUI method code correctly, it only works for real
> GUIs, not terminal emulators that happen to be GUI
> applications, right?

No, they work for text terminals as well.

> At least the window-system variable, which gui-method apparently uses,
> is always nil in my tests with hterm. Should this be something
> like 'xterm?

No, it will be nil.  Of course, it will apply to all terminals, so
you'll then have to check whether the current text terminal is an xterm.