It is not possible to add to this list of options on the drop-down menu.
A workaround would be to create a new Event Log report in the Orion report writer. In the Filter Results tab, create a new elementary condition and specify the custom property you wish to filter on.
If you are unsure how to create a new report from scratch, you can use one of the existing Events reports as a template. Tick the checkbox on the General tab to Make this report available from the Orion Website.
Thanks Paul - This works fine - wanted it as a check/balance against another report for availabilty and everything adds up