I see Begins With, End With, Contains and Exact. How can the user bring up "empty"?

That would be a trick one, since if you are in a text field, empty would mean that you don't want to search on that field.

Thinking about it, if I were to do something in that scenario, I would probably create a "convention" in a way to tell my user that if they wrote something like "(empty)" or whatever string we would choose, I would then check for that in the BrowseDialog BeforeSearchExecuted and manipulate the QueryInfo argument to make sure the search engine fetched the empty values.

Just an idea, not sure if this is what you are looking for...

could you provide some sample code to do this?