It sounds like a good idea to have the freedom to get our charas affected by recurrent effects at random, though I kinda share everyone's worries about people getting bored of it soon/not updating if it's not for that. 8|a
On the other hand, I think come this point all charas have their own plots going and they would keep to them with the optional events as a side resource, if that makes any sense. Maybe the optional events would come in handy for helping out those plots too.
And I trust everyone's good judgment not to overdo it. Like, if we got a Fujimoto I don't think Azie would play the personality switch event with him for his intro post and the like.
Restricting it to once in a month is a good idea IMO. But wouldn't it get tiring for you to keep watch on everyone and see when a new event would fit among the recurrent ones? Maybe going for the announcing new events beforehand would be best?