Brabus have done this properly but not aware of any custom jobs.
Personally I think it'd be a waste of time and money to bother with something like this. Practically everything around that V12 would need to be heavy duty and performance putting this build into silly money. It would have also made more sense to start with an E500 and not an E320 due to the larger body and engine bay modifications giving more space. Would the wiring from an early 600SEL not work in a W124?
Now that rust has set in I don't think it's a project even worth pursuing unless you have a money is no object attitude towards it.
Leave the V12 in the S class and enjoy the V8 in the W124 (500E/400E) !
1991 sl300-24 project,s124 280,w115 no interior, w108 project no engine
I have seen one of these - a coupe - which runs (although I never witnessed it at the time. It was waiting for the loom to be tidied up etc, the guy was a crackpot (but a good one if you know what I mean !!!)
Out of interest - what do we think it would be worth (or what someone be prepared to pay for it if it was sorted properly and tidied?