Several braille display drivers add emulated system keyboard keys to NVDA, such as escape or alt. It might be an interesting feature if one could add other keys for emulation, for example when one wants to add an emulated system keyboard key to a braille function key which they can't choose by default.
I'd suggest adding a separate button to the imput gestures dialog "Add emulated system keyboard key", which allows you to press a random key on the keyboard which is than added to the emulated system keyboard keys category in the dialog. After that, one should be able to add a gesture to it.