We hear you loud and clear! We know this is a frequent ask and we do have this on our backlog. Unfortunately, I don’t have a timeline to share right now.
We are in the process of updating docs to include Hybrid Azure AD join as a supported scenario in a single AD forest to multiple Azure AD tenants. This could be achieved using client side SCP settings that can be configured using GPO. However, there are certain limitations with a single AD forest to multiple Azure AD tenant setup. Capabilities like Windows Hello for Business using cert trust deployment model, enabling Conditional Access for on-prem apps federated with AD FS, Syncing Office 365 Groups back to on-prem Exchange, enabling Seamless SSO and enabling Azure AD Password Protection for on-prem AD DS will not work.