Hi Stefan,
if you want to do some development, the following should work:
1. Create a new Z-Searchparameter (value parameter) for the search (for example ZEMTPYEMP, type FLAG)
2. Redefine the method that does the search
3. At the beginning, remember your Z-Parametervalue and delete the parameter from the search options (since the standard search can not operate with this parameter).
4. Perfrom the standard search (call the super-method)
5. If your parameter was set to X (show only empty employee responsible), loop over the result list and delete all messages who have an employee responsible maintained.
Unfortunately is not the best solution regarding performance.
But the SAP does this herself in the solution manager widgets.
Hope this helps,
Christoph