Yeah, this is a common problem. The only solution I've been able to find is from Cemaphore (http://www.cemaphore.com/index.php). They make a utility which can perform a 2 way sync on users on premise and online mailboxes, so if a user has a calendar entry in their on premise mailbox it syncs it to their online mailbox and vice versa. A solution like this would require a drastic change to your migration and would incur additional cost, but it can be done. It's probably easier just to migrate everyone to Microsoft Online as quickly as possible.
Chad