Hi Scott_Bumphrey,
The key part of your query is this as you mentioned - "to create a row for each of the main accounts that fall under the Staff Costs Category and not have to manually create a line for each one".
Yes, this is possible and you only need to link "Staff costs" related main account category in the Row Definition.
Once the report is generated, you will find the staff costs related total value is showing in the report provided that you already have posted transaction records in the system. You can then click on that total value which will appear as a "Link" type. It will then drill down to the posted entry level. And once it drills down, you will be able to see multiple staff costs related main accounts and their posted values.

But in order to get the staff costs related total value as a type of "Link", you have to do this setup in the "Report Definition" first.
Just open the Report Definition, and select Financial & Account option in 'Detail level' section. Save it and then generate the report.

Financial - It will only show values in the report but no drill down option will be available.
Financial & Account - It will help you to drill down insight of the value shows in the report.
Financial, Account, & Transaction - It will give you more layered drill down option even to the posted transactional level. Which means, once you drill down at every account wise posted value level, you will further have the option to click on each and every individual main account value to see the posted transactions which is summed up to show the value in report for that main account.
The 3rd option above will work like this.
Staff costs = 1000.00
You click on 1000.00 and it drills down to next level where you will find staff costs related main accounts like 1001 and 1002. Each of these account have their own posted value.
Now you will have further option to click on those values which will also appear as a "Link". Below are the options you will get.
-- Drill to report transaction level
-- Open account transactions
If you click on Drill to report transaction level, you will be able to see the posted transaction detail in the report itself.
Else, click on Open account transactions. System will take you directly inside to the posted voucher transaction in system with that main account.

Kindly check and provide your feedback.
Regards,
Sourav