I found my own answer, but posting it for anyone else who may have a similar need.
In the AdvDevGuide.pdf documentation (page 20), the solution is/was ....
Using negation on properties
EQL supports filtering by the absence of assignments on records. By using the not operator, you can
filter down to the set of records which do not have a specific property assignment.
returns all records with the "author_id" property, while:
returns all records without the "author_id" property.