Manage custom security attributes in GCC High

In order to allow guest users in our GCC High tenant each guest must have a sponsor from our internal users. I created a custom security attribute called Sponsor. I can manually assign and populate that attribute for a guest using the AAD admin portal. We have around 1000 guest users so maintaining and reporting on this attribute manually is not practical. I don't see a way in the interface to query on the Sponsor attribute, and I don't know how to populate or query on the attribute from PowerShell. I've looked at Graph Explorer, but it does not seem to work correctly in GCC High. Any suggestions are welcome. If anyone has the PowerShell code to do this that would be great.