I have a story with a complicated timeline, so I’m tracking “Timeline Date” as custom metadata. I have a saved search matching the regex “\d{4}” to find all 4-digit dates (ie, years). I can sort by that metadata field in the Outliner to see everything in chronological order, which is great! 
But the other piece of the equation — finding any documents that do not have a timeline date set — is eluding me. It doesn’t seem possible to search with for something like “isblank:timeline date”.
So my feature request is A) searching within specific metadata fields, and B) searching for blank fields.
Thanks!