Manual focus lenses where used for many years on SLRs. The only thing different about focusing a manual focus lens on a DSLR is the focusing screen. But Canon offers the old-style focusing screens for certain camera bodies; these show the actual depth of field even at f/1.4. That's all that's need to make the Otus work well on a DSLR.
The resolution of current digital sensors exceeds that of film, particularly when you consider the 'typical' print sized enlarged from 35mm negatives vs. print sizes easily possible with a ≥18 MP digital file. That means slight focus errors that were tolerable with film are often unacceptable with digital.
Also, Canon doesn't really offer 'old-style' focus screens, they offer 'not-so-old-style'. Currently, you can get a screen with without the same degree of laser microetching for brightness - those are the 'super precision matte' screens that show you the true DoF of fast lenses…or you can get screens with manual focus aids (split prism or microprism) but they're based on the stock screens that don't show the true DoF of fast lenses. When shooting film, there were focus screens that both showed the true DoF of fast lenses and had the split prism/microprism collar focusing aids.
Though you are right, it is still possible to get those old focus screens on DSLR through specialize web site. I use a Canon EC-B focus screen (split-screen focus aid and precision matte) on my 6D and it works quite well. The biggest problem, for me, is that the viewfinder is still much smaller on DSLR than on good old bodies, such as the Pentax K-1000. Manual focus lens are still a joy to use with the proper screen and some practice, although it is not for everybody.
Where do you get that focus screen for a 6D ?