Question/Idea: Focus balance on manual SLR lenses

One of our readers Daniel asked us a question:

There are quite a lot of manual focus lenses out there from Zeiss or Voigtlander or via adapter for DSLR. Would it be a big thing to implement a focusing balance into the viewfinder of a DSLR (maybe only a firmware issue)? A focusing balance would provide more information about approaching focus in the viewfinder. The current confirmation only shows in the short moment, when focus is there and can easily be overseen or overrun.  

focusing balance = horizontal line where focus is fixed indicated in the middle of this line and a moving arrow on the line shows, how far or close focus is. If the moving arrow is on the right side of the fixed focus point, the objects focus is farther away and the focus ring therefore has to be thrown to the left according to the distance of the arrow to the fixed focus point. And vice versa for an object with closer focus. 

I think, this was there in analog-times already and would make manual focus lensens even more attractive. In fact a DSLR would get the same focus orientation like a Rangefinder has with its rangefinder windows. 

Well... In fact we already have it. It is a split-image focusing screen.

but I like this focus balance idea for several reasons: You can use off-center AF point, you can use full size matt-screen, and typically a matt screen with split-image aid has larger DOF, also in principle you can obtain  distance reading between the object and the focused plane.

In theory implementing it is definitely plausible.

All the SLR's today use phase detection auto focus, that is there is a module that tells how the light rays enter the camera and if they are in focus, if not, which way to turn the lens to bring the in focus and by how much.

That's what the phase detection module in every SLR does. All the information is there, and used by the camera automatically. So is it a big step to make this info available to the operator behind the camera? I guess not. The analogy I have is the car that you are driving daily (assuming it isn't a classic), it's got dozens of sensors reading stuff like temperature, oxygen, air flow, fuel mixture, and what not. The data are then fed to the ECU, which determines how to run the engine automatically without the operator concerned over the readings. There are people who hacked their car to read those sensor readings, so why not for cameras? 

I am not saying one can get a focus balance from a SLR with a simple firmware "hack", my point is all the information is there already, it isn't a big step forward to present them.

Now if the manufacturers will implement the focus balance feature is another question. I don't know how Zeiss struck the deal of producing lenses in those proprietary EF mount, F mount (perhaps Zeiss told those Canonikon: oh! don't worry, nobody likes manual lenses, our Zeiss lenses in your mount will not take away sales of your oem lenses), but implementing this feature that aids manual focusing indeed encourages the use of manual lenses. While letting Zeiss produce lenses in proprietary mount is more like Zeiss lives or dies on its own, encouraging manual lenses is a totally different matter.

In fact, it would be more reasonable if Canonikon discourage the use of manual lenses. We received a note from Dave, who reported that the focusing screen on the new Canon 5D mkIII (mark 3) is fixed, hence cannot be changed to one that optimizes manual focusing. If this is true, it looks indeed like at least 1 player doesn't want you to use manual lenses. Or perhaps Dave had a pre-production 5D mk III?

Could you folks verify this for us?
Canon 5D mk III having a fixed (non-interchangeable) focusing screen?

Update 1:
Focus screen on the Canon 5D mark 3 is fixed!

Specification page at Canon USA.

http://usa.canon.com/cusa/consumer/products/cameras/slr_cameras/eos_5d_mark_iii#Specifications

Quote: Focusing Screen Fixed 

Update 2:

Markus from photozone.de noted that Nikon dSLR's do tell you which way to turn to bring the lens in focus. Given Nikon's attitude toward compatibility with its manual F mount lenses, Nikon *could* go a step forward to give a distance read-out between in focused plane and out of focus object, which will require knowing the focal length for instance. Canon obviously isn't interested.