You can use netcat (a.k.a. nc) to listen on a socket and respond to requests. Basically you just create a script that pipes an echo of the domain policy to nc. e.g., create a text file that will be your script, say, domainPolicyServer.sh:

#!/bin/sh
while true; do echo '<?xml version="1.0"?>
<cross-domain-policy>
<allow-access-from domain="*" to-ports="1-65536"/>
</cross-domain-policy>' | netcat -l 843; done

You'll need to make it executable, and you'll probably want to make sure no one messes with it, e.g.

chmod 700 domainPolicyServer.sh

You can then start that with:

/path/to/file/domainPolicyServer.sh > /dev/null &

That will start it in the background and keep it quiet.

You can also add that command to your /etc/rc.local to ensure it is started at boot up.

This works, but I'm no Linux guru, so feedback is welcome.