You are correct that this is unsupported. This is mentioned in our documentation here: https://learn.microsoft.com/en-us/azure/active-directory/app-provisioning/customize-application-attributes#editing-the-list-of-supported-attributes
Our GSuite connector uses a proprietary GSuite API, and we in most cases do not support using custom attributes with connectors based on proprietary APIs. We do support custom attributes for any SCIM 2.0 applications, but that isn't helpful here, unfortunately.