Reason for wanting to use and Actus or Universalis is that you gain the ability to use rise and fall along with shifts to stitch multiple images together with no errors and more precision. As well as adding tilt or swing independent of those movements.
ok, but then, you will run out of image circle... nonsense !
With ultrawides on MF, even with dedicated real MF backs, thing are not that simple !
For example, the shortest lens available for full frame back (using movements) is the rodenstock 32mm !
I have the rodenstock 28, and i'm using it with a 36x48mm back... but movements are limited !
There is a 23, but nearly no movements available...
If it was that easy to make a 17mm that covers MF with movements, Rodenstock would have done it...