These days they can make transparent monitors, would it be possible to make transparent sensors?
If so, we could get mirrorless camerras with OVF. Place the VF in alignment with the sensor, and voila!
What is life without ambitions, right?
Not so much ambitious as pointless...you'd be able to make an OVF exactly the same as cameras in the 1840's! A viewing screen showing an upside down image. You could add a loupe to make it possible to bring it to your eye, then mirrors or a prism to correct the image orientation...
Everything to do with the art of photography is a guideline. The only rules are generally enforced by the police.