Any way to put a diagonal line though a cell?

Yes,

You can add a sketch entity (line) to run across the diagonal of a cell. It is a manual process in dwg mode. If this is something you do often, I would create a drawing symbol for each cell size needed and drop the symbol. I don't think either of these methods are anchored to the table so if the table moves they will be misaligned to the cell.

It is probably not the company standard, but could you add a hyphen "-" instead to designate this or use a back slash "/"?