Classic EAC to the rescue. The old migration wizard correctly defers to autodiscover for the server information. I was able to connect for a test migration. Sadly, even after changing my test account to "onmicrosoft" to clear the "proxy address is already being used..." error, I receive an AD sync error because the alias is still the same, just like when trying to create the same email address on two different domains. Microsoft trying to save us from ourselves again. So it looks like the answer is "No, it can't be done." Apparently the users must be completely removed from Azure AD before attempting the migration due to this bug.
Thanks to everyone who attempted to help.