Just uploaded version 1.1.1 (1.1.0 had a small issue with a coding mistake >.>) which adds a button in the "shortcut button" area. It's a toggleable button so it will be in the "toggled on" state when expansion is enabled.
Adding buttons to the shortcut place seems to be what is recommended in new mods now, rather than placing them in the top left as is otherwise common (though it does mean it won't be located in the same place as the toggle peaceful mod).
One minor caveat is that if the expansion setting is changed elsewhere, the button won't refresh and show that (such as if another mod changes the expansion setting), I couldn't find a way to detect when that setting is changed outside of the mod.
