I can see what you're trying to achieve, but by restricting the outgoing port there will always be a potential for failure.

Say for example your outgoing connection for the XML 'pings' was set to always use port 50000 (and it was working :-). You do not have control over the ports that incoming connections to your MT server will be using (i.e. the local ports on computers browsing your site), so there is a chance that an incoming connection could be using port 50000 at the same time as MT was trying to open an outgoing socket. Your XML 'ping' would fail.

Would it be secure enough to make a list of the IP addresses you need to XML 'ping' and then allow connections from any local ports (in the range 1024-65535) to port 80 on the specific IP addresses, e.g. 10.0.0.1:80, 10.0.0.2:80 etc? Then your OS can still allocate outgoing port numbers in the usual manner.

JJ