You can add an external display or EVF to a DSLR using the HDMI or USB port. These are available and are widely used for video. Two weeks ago when I was at Pori Jazz, I saw one Sony mirrorless camera with 70-200/2.8 Nikkor attached, and the videographer wasn't using the EVF; he had an external display mounted on the hot shoe and he was using that to monitor the footage. There was also a Red Dragon with EF 70-200/2.8, it seems adapted lenses are really common nowadays for video work.
A "hybrid" OVF/EVF would result in a reduced quality viewfinder and thus I would not be happy with such a contraption. The reason for the quality loss is that the EVF would be positioned at the front of the prism and thus the coatings would have to be removed to permit the light from the EVF to reach the ocular on the other side of the prism. Removal of the coatings would lead to loss of light when using the OVF.
A better solution is to have interchangeable viewfinder modules as was available earlier in 1-digit F series Nikons (until and including the F5). However, this also apparently results in some loss of viewfinder brightness;; according to Nikon this was the reason why the F6 was made without interchangeable viewfinders. I never owned an F6 but the F100 viewfinder was appreciably brighter than that of the F5. I suspect the focusing screen was a part of the reason; the F5 viewfinder was easy to manual focus with but the image was a bit dimmer.
I think it's better to just use mirrorless for those applications where mirrorless is advantageous and correspondingly for DSLRs. A hybrid isn't necessarily the "best of both worlds", it could be seen as the "worst of both worlds" depending on what the photographer values most. A hybrid camera with reduced viewfinder quality but increased weight (due to having the components for two viewfinders, only part of which would be shared) would not satisfy me. I would prefer to keep mirrorless mirrorless and DSLR DSLR, and if necessary, maintain two systems and choose depending on the requirements of the task. One of the main advantages of mirrorless is smaller size and lower weight, and perhaps the main advantage of DSLRs is the high-quality optical viewfinder. A hybrid camera would give neither of these benefits.
I totally understand the benefits of silent shooting and once the electronic shutter is fast enough to minimize rolling shutter, I will likely be happy to purchase such a camera and lenses for applications that require silence, but it won't be my main camera due to the lack of OVF. I generally think of mirrorless ILC cameras as just one type of camera among others and having this choice is important to users. But not necessarily in the same camera. Of course, if someone wants to make a hybrid camera they're welcome to do so, and then we can evaluate it on its merits and drawbacks.
