How to send system messages to irssi?

I use Irssi together with bitlbee as my overall communication interface (IRC, GTalk, Twitter, XMPP …). Whenever you can speak of something being a message I try to bring it into irssi. Up to now my setup uses libnotify to forward system messages to i3 wm, but considering the basic approach described before I would also like system messages to appear in a window in irssi. I tried to figure out how this could work but still lack a general approach.