[Plugin: Custom Query Fields] All posts where the custom field is not empty / null

Hi, thank you for this great plugin! Is it possible to have a query where it just pulls any post where the custom field is not empty?

A la:
mysite.com/?cat=56&city=>0 (greater than 0, greater than null)
mysite.com/?cat=56&city_min=>0
mysite.com/?cat=56&city_exclude=NULL
mysite.com/?cat=56_all

Thank you!

http://wordpress.org/extend/plugins/custom-query-fields/