Hi Nic. I don't know if you can do it exactly like you want... but try this:
On the Categories and Items tab, open the settings page (click the gear icon) for your main category and make sure the "Aggregate only non-empty grades" option is checked.
Then, on the Settings tab of the gradebook, in the User Report section, you can control exactly what students see (or don't see) in their report.
To verify, choose "User Report" from the drop-down menu in the top-left of your gradebook, then select a student's name... and you can see exactly what they see.
Others may have better suggestions, but this is how we normally do it. Good luck!