I find myself wanting to tell someone something who is offline currently, and it's a lot of work to mail a book. I wish there was a way to /tell someone something that's offline. (maybe simply allow us to mail nothing? /mail samtheboo Hey, what's up? would deliver samtheboo Hey, what's up)
And there's always the forums too. I like your idea but can't help agree with Chickeneer too that we already got quite a bit of options here.