Hybrid confusion after linked mailbox deletion

Hey guys,

I got a tricky one, hopefully I can explain it in the needed detail. So I'm in the project of consolidating several organizations in one forest, one domain. I'm just referring to company A and B including target domain C.

So company A got an Exchange hybrid deployment in a dedicated forest. The smaller company B (different forest) decommissioned their Exchange server and received linked mailboxes at company A. Not that complicated yet, but the admins have been confused now, because the linked mailbox accounts and the original accounts have been synchronized. Over the years the knowledge was lost.

No we are going to build up target domain C. The linked mailbox accounts of company B have been deleted. The original accounts and clients have been migrated to target C and the objects are synchronized with the most recent information to Entra.

So now the mailboxes of company B cannot be managed in Exchange Online, because they are synchronized (Consistency-Guid has been transferred), but the on-premise object has not the proper Exchange information. I know in general you can execute Enable-Remotemailbox again on a provisioned mailbox in order to link it again to an on-premise object, but this only works in the forest, where the local Exchange Server is homed.

Is there an easier solution than deploying a second hybrid server in target C?