There's a Adium plugin, pipeevent (Github), that lets you send stuff to Unix commands.
Adium XTras page for this says the plugin has been banned, so don't try. Just download it from the Files section of the Github page above
Original blog entry announcing this
People report issues with Adium crashing on message receive, but I don't have this issue (Adium 1.5.3)
Note: this works just fine with the Mute settings described above!!!