I thought about this a few times too. Not all casings have those large holes cast in them. I just check a UN1-13 (1988 case) and a 369 (1979 case) and they dont have the same casting holes in them. I have a few UN1's in the workshop, and looks like from 1989 on they put those holes in. Dont know why. Mind you, there are also different mounts on them according to the year as well.
Also doesnt really look like there is going to be too much room before the disc would hit the bellhousing. I just tried a 10.5" disc against one and it would work. Heres the problem though, as the disc would be quite a way out on the CV joint (if you're using the tripod CV) the leverage against the 'bearing surface' would be rather large. As the output shafts only run metal to metal you could wind up with a lot of wear at this point.
I dont think there would be too much to gain from having inboard discs.