What's the point of enabling it if there's no corresponding functionality in EmulationStation ?
This would be a pre-requisite to allow extra functions as currently 12 buttons are used to control ES and there are no "spare" buttons.
provided [hotkey] doesn't trump an existing input
Yes, hotkey can have three states: Absent, present or mapped to Select.
Case hotkey is mapped to Select button:
The Select is used for Start Screensaver, choose next item in screensaver media or bringing up the Options dialog in a gamelist to my knowledge. These events are consumed before the systemcarousel and gamelist logic and do not reach the later components in the GUI stack where hotkey would be processed (e.g. scroll a game's description with hotkey held and joy up/down pressed as one possible ES enhancement).
Case dedicated hotkey button:
Each GuiComponent would decide if it would act on hotkey input. In turn this also means a hotkey-button-combo can only be processed by one gui component when there are multiple gui components displayed.