I recently posted an idea on IdeaJam to enhance the mail rules dialog on a Domino server to overcome the restriction of only use the “hard coded” conditions. There have been several attemps to enhance the dialog in the past ( Chris Linfoot for example ), but none of them adresses the use of a formula as a condition.
I’ve modified Chris’s sample db and added the requested functionality. You can download it here. To use it in your environment, replace the (RulesDlg) form and the Rules Script Lib in your names.nsf with the according design elements in the download.
Keep in mind that modifying the Domino Directory is dangerous if you are not sure of what your doing.