It's backwards compatible in that the users with no HOTKEY fields in this file will not notice any difference, the hotlist will work as usual for them.
The only problem is that the user must select hotkeys so that they do not clash with the keys that activate the buttons at the bottom of the window. However, if this happens, the list hotkeys just get the priority and nothing bad happens. For example, I have used "c" as a hotkey in the list, so now I cannot press "c" to activate the "Cancel" button - but I don't need it anyway, so it's not a problem (and when I need it, I can go to it using Tab).
I use the hotlist a lot, and with hotkeys it became much "hotter" and easier to use. So please discuss this, and if interested, I will post the patch for inclusion into mc.