B2B SPO: GCCH tenant members as guests on commercial Entra

Has anyone successfully enabled GCC guest access to a Commercial SharePoint Online site?

We support customers on GCCH tenants and are migrating an on‑prem SharePoint workload to Commercial M365 SPO. Inviting GCC users as guests fails with token/Token ID errors. Entra sign‑in succeeds, but SharePoint token issuance fails. CTAP is configured on both GCC (outbound) and Commercial (inbound).

Microsoft support indicated it may not be possible (“Entra won’t pass OIDC to SPO”), but Microsoft documentation suggests B2B for SharePoint works across US Gov and Commercial.

If you’ve made this work (or confirmed it can’t), I’d appreciate any practical guidance or gotchas. Thanks!