After exact 364 days the irssi guys published my last screen_away.pl version which includes SILC support.
You can find the script here: http://scripts.irssi.org/
For anyone who don't know about screen_away: if you run irssi in a screen(1), this script set's you away if you detach the screen session and cancels the away status if the screen is reconnected. You can configure the away message and optional also change the nick (but ppl usually don't like nick changes to show the away status).
Install instructions are in the head of the script.