Yes, that would work. Create two policies, one for on premises (without dynamic delivery) and one for cloud (with dynamic delivery). Policies rules are to apply the cloud policy for members of a group that you populate once you move the mailbox to EXO. Other (second policy in order), apply to all users. First policy will apply to cloud mailboxes.
Once all users in cloud remove first policy, and the group if now not needed, and enable dynamic delivery for that policy