I think it's a good idea! Re: the ruling: maybe make it more flexible? Like, it should be about once a month (because you really should play your character outside of events too), but if someone happens to play in two events this month and none next month, that works too? (Personally, that's how I do that kind of thing at CFUD -- it's not a rule, but most people are usually sensible enough to space out their special effects.)
On that note, I think having the events available at any time means that people will end up using them when they're most interested in playing a particular effect, rather than everyone doing an event all at once since they won't get a chance to do it later. Which is a good thing, I think.