Another way to increase unhappiness is to built lots of minimal ships and then use the "Fire On" option on them.
But, does rioting actually have anything to do with the chances of a planet breaking from the empire? I thought that was an unrelated random event.
If you don't mind modding, you could of course add an intelligence mission which causes the defection event... but you could probably only target enemy planets with it. You could temporarily hijack another empire to use it on yourself, though...
PvK