I think in this case you still couldn't claim it was certified. It would be on users to discover that if they plug an HDMI capable screen into that HDMI shaped port on your widget device, things just work and video shows up as expected

Note that if the protocol itself only works if the device claims certification you may be able to claim certification in the protocol. However you couldn't claim certification in marketing or any other context except where things wouldn't work if you were not certified.