To the devs:
Doing this seems like a good solution to me:
Use the shortcuts[] property to show the keybinds next to commanding menu items, but also add the assigned keys from shortcutsAction to this array.
E.g. in RscMenuMove we have:
shortcuts[] = {2}; shortcutsAction = "CommandingMenu1";
if the user is using Numpad1 for "CommandingMenu1", you just add it next to the shortcuts[] array before creating the menu (in this example, shortcuts[] = {2, 79}; should be used before creating the menu).
I have tested this with custom commanding menus and it works perfectly fine.