Sorry, I did not provide full picture. After this discussion on devel I've linked, I spoke to Michael what changes would be in compliance with EDK2 driver model guidelines. An agreement was reached: Root Bridge driver that implements SIO Protocol and Serial I/O driver (with new entrypoint) that depends on SIO by DEPEX.

This way, driver would be a DXE Driver that runs new entrypoint only when SIO Protocol is present. I believe this approach doesn't violate UEFI Driver model rules since it uses standard DXE methods.

Also, the approach of modifying driver's entrypoint was suggested by Zhichao Gao.