Can you live with re-arranging the view so one of the dimensions is on Columns, and the other on Rows? That should give you a layout where you can show a 0 for the missing values.
Thanks Matt, I know this would be the easiest answer but it isn't quite what I want in the viz.
I know that I would need to do some serious data manipulation to do this so the person requesting this has since said it isn't as high as a priority as first thought!
If there is however an easy Tableau check-box which can magically do this for me in the future, please do let me know!