In a previous post, Hosting a Subset of Your Mailboxes in the Cloud, with FOPE Protection, I talked about documentation that addresses a cross-premises mail flow scenario where you have FOPE protection for all mailboxes. To clarify, you host a portion of your mailboxes in the cloud and a portion with Exchange Online, and you have FOPE protection for every mailbox. My colleague Micah LaNasa just released a video, Configure a Shared Address Space with On-Premises Relay where the MX record points to FOPE, that shows how to configure your FOPE connectors for this scenario.
We have also released other videos that focus on FOPE configuration, such as Overview of Complex Mail Flow Scenarios in FOPE. We hope you find them helpful.