For what it's worth, I have a database that seems to do what you wanted. (I have in AC2K but can convert. Have zipped but file still 267KB so can't attach here - if you want, please email me)
This database has a form called "Entry Form". On it there are lots of combo boxes where you can choose criteria for selection or leave them as a wildcard (*). There are three command buttons - the first one opens a form that uses the selection criteria in the SQl in the record source. The other two buttons open the same second form but add additional filters on top.
I don't think this will deal with null values in fields - sorry.
I have learned VB and Access on-the-fly the past few months and so please excuse the poor naming standards, etc. - a bit embarrassing. I have now bought a book so am improving! However, it works very well and someone might find it useful.