I appreciate this is not ideal, but when a remote client connects to Airfoil Speakers on my machine, I see a line similar to the following generated in the Console, coming from
Firewall: Allow Airfoil Speakers connecting from 192.168.1.72:52013 to port 5000 proto=6
So potentially you could
appfirewall.log looking for "Airfoil Speakers" using something like
cat /var/log/appfirewall.log | grep -i "airfoil speakers connecting"
and parse out the IP from the relevant lines.
Alternatively, I notice that Airfoil supports AppleScript so maybe there's something you could do with that. Or you could drop Rogue Ameoba an email, I've always found their support to be excellent.