The hard part about that is literally having it fit in the engine bay and finding a transmission that fits. If V12’s follow the same BMW principle as BMW’s I6’s (that the transmission mating patterns are the same), there is someone who offers an adapter plate for a transaxle for the old 8 series V12. I agree though that would be an insane insane project.
For S55, you may like this (however this is likely standalone):
https://www.instagram.com/p/ByQtRmeA0Mv/
I feel the electronics are what scare people from even imagining these projects, but all they really want is the CAS and DME, and the emission equipment would work as if it’s the original car. You’d need to make a custom mount to weld in some plates for engine mounts, maybe massage the frame rails (as was necessary for the N55 in E36), but that’s really it.
To be clear, I’m a 21 year old business student that hasn’t done an engine swap, so put me in my place if I’m wrong. But I’ve witnessed some insane engine swaps online and in person. And even though I’m in finals week, I spent about 35 hours last week reading about these systems as I was looking into retrofitting “active seats” and doing further research on my comfort access E9x keyless start retrofit into my E46.