Hello @Sam Campbell,
Unfortunately, Entra ID dynamic groups cannot be built by querying employeeOrgData fields i.e., attributes like Cost Center are not accessible in Azure AD dynamic group rules.
Please find the properties in the below document which are supported for Dynamic Groups
The only workaround at the moment is to create custom attributes to users and make the value as the value of the cost center attribute and use it for dynamic groups.
Unfortunately, we had hit the limitation, we always look to improve and would love to hear your thoughts. If you have any ideas or feedback on the feature in the question, please let us know your idea in the below given portal and our development team would be constantly monitoring them. https://feedbackportal.microsoft.com/feedback
If this answered your queries, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".