Yes, it should work, but it may depend on whether its a Extension Attribute or a built in field. I know with EAs, you can easily search for Macs that have no value in them (i.e, they haven't reported in, so no value yet)
just don't enter anything in the field when you add it into your Advanced Search. Make sure to add in the column to your display fields so it will show that.
I have never tried this with any of the regular items Casper reports on because I think they generally will always have some value. but I don't know for sure.
