Hi,
The tool wasn't written inside Wireshark because it started off as a dissertation project where the GNU General Public License was problematic for the sponsoring party. Since then the third party have agreed to release the IP to the authors, allowing it to be released Open Source.
The application is used for analysing large amounts of data (>500Mb) so re-starting Wireshark (although not impossible) would take too long each time the user wants to update the filter.
Thank you for your advice, I will look into the Wireshark Lua plugin.
Thanks,
Lee