Something I had just posted to the maillist:
Once you write your Custom Filter (remember to give it an unambiguous description) and use it a few times, why not share the Filter definition with us?
The easiest way to share a custom filter is NOT to describe all the rule settings in excruciating detail. Instead, just find the custom_filter.xml file in your Gramps User Directory. Open it with a text editor and share the XML snippet. (Finding the rule start & end points, buried among XML tags, is easier with those unambiguous descriptions.)
Interested parties can paste the snippet (after making a backup) into their own custom_filter.xml file. The next time they open Gramps, it’ll be in their Filter list.
(And, if you unfortunately lose your Filter definition in the future, it will have been preserved here in the Maillist archive.)