gilir on the awn forum wrote a patch to Liferea to set awn:

gilir's message

I'm not sure what happened with this.

Here is another approach that uses the Liferea Lua scripting system:
http://www.stupididea.com/liferea-awn-plugin.py

Use with a Lua script in Liferea:
num = liferea.feedlist_get_unread_item_count()
cmd = string.format("%s %s", "python /path/to/liferea-awn-plugin.py ", num)
os.execute(cmd)
connected to hooks startup and item unselected. By tristil (Joseph Method) with help from tsukasa.