Hi Tim,
I'm not sure how to select specific cells vs an entire row or column. In playing around with your idea, my edit session will only allow me to select an entire column. Also, when I tried the Field Calculator ([Field]="Null") it left my <Null> cells the way they were, and any cell that contained information was assigned a number (0) so still not the <Null> value I'm looking for.
I have tried simply changing the cell to a text "<Null>" but the query I'm running (currently on Symbology to display a value based on a <Null> option but I would also like to display data based on a definition query- [Field] IS NOT NULL) but it seems the text "Null" is not read by ArcGIS the same way as the coded <Null>.
I hope this all makes sense!