Hi savvyxeus,
Currently, mailbox permission cannot be migrated during the cutover migration. We need to configure the mailbox permission on Office 365 side after the migration is completed.
However, what you mentioned is great. And I can fully understand that it would be nice if the mailbox permission can be migrated to Office 365. Your idea is very useful and it will improve Office 365 services. I will write a detailed summary and deliver this information to our product team. You can also submit comments to the feedback collection team. Many Office 365 features have been designed and upgraded based on customers' feedback.
Thank for your tine and efforts.
Thanks,
Young Yang