@wib
Never say it's impossible, 700D and small cameras give huge proof of concept about the possibility.
-So why it's not there yet on 5D3?
5D3 share some of the registers I used on 700D, but the rest and most of them are different and some of them are not presented in 5D3, but that doesn't mean it's totally different concept, well, actually we don't know yet, and this brings us to the second part of the answer, No one tried to understand the function of the new registers which presented on 5D3, it may has the same effect like the 700D ones, but in a different names.
Currently there is no active Developer or Contributor is working on 5D3.
-Would I work on 5D3 to achieve the new previews?
I have the ability to work on it (ability in this case means the good skills, time and the inspiration), So yes, but I don't have the main tool which is the 5D3 Body in my hands,
Open Collective is a solution for this problem, if 5D3 users want me to work on it, in time no one worked on it, I can apply on 5D3 body, and get a used one.
Open Collective or other central donations for ML project currently work in progress.