If you don't have a 'retropad A' button defined, of course it's not going to work in the menu . My remark was:
How the button is mapped to the underlying core doesn't matter for the menu (RGUI or otherwise).
I agree, but the reason that I don't have a retropad A button defined is because the core doesn't use it for it's logical function in the mapping. I could have had both [N64 "A" and RGUI "A"] if the core used different bindings.
Obviously this is a minority problem for people trying to use pads without the full set of retroarch inputs, and I can still navigate the RGUI using a keyboard in my case :)