Yes, its working!

Thanks for all the suggestions.

For future reference: the only cfg-files that matter are in ‘/opt/retropie/configs’
In my case I added the two lines to ‘/opt/retropie/configs/all/retroarch.cfg’

(My ‘/home/pi/RetroPie/configs/retroarch.cfg’ was simply ignored)

I also set ‘input_enable_hotkey = tab’
(Perhaps that triggered the ‘input_enable_hotkey_btn’ to work … I don’t know)