Found this from gemini subforum, even though mapping to silver button doesn't work but you can use any normal key combo, just assign one of the key combos in /usr/share/X11/xkb/planet_vndr/cosmo value Pointer_Button3 and then run 'xkbset m' (can also assign to left click as Pointer_Button1 which helps for SDL apps somewhat, in battle for wesnoth buttons in main menu can be clicked this way, though unit movement is still broken, maybe key release is not issued?)
Just be careful editing this file, as you could possibly make yourself unable to enter root password/login etc, best to edit some layout you're not using and switch to it and back if something went wrong, as that needs no key presses
