http://mircscripts.defcon-one-script.de/tmp/msgext+.zip

This is a new version (v1.4a) of my Message Extensions Addon. It includes a feature called Content Filter which will do what you want. Even if it is not finished yet it should work for you.

If it gets finished i will update it also on mirc.net:

http://www.mirc.net/projects.php?go=1097394278

Note: Version 1.3 (mirc.net) do not support that feature.