Hybrid will not help in this specific scenario, but generally speaking every configuration involving dirsync requires you to have at least one Exchange server on-premises for management purposes. As you've noted above, the on-premises AD is the source of authority in such scenarios and changes need to be made there, and the only supported method to create/manage Exchange-related objects and attributes is via the Exchange tools.
If you dont care about the supported bit, you can do fine without an Exchange server, however at the very least you should extend the AD schema with the Exchange schema extensions.