Could be an interesting conversion for something like a 500SLC or other 107.
Expensive though. Would also be interesting for W113 but might hurt values due to non originality. Having said that, a good 5 speed conversion does not hurt (and often helps) values of classic jags, especially those first equipped with the Moss gearbox.
I am not seeing it so much on the S class saloons. I'm a big fan of manual transmissions but a big luxury car like an S-class seems to fit better as an automatic. For example, I am not sure I could see a 6.9 with a manual, but I'm sure there would be people who would want that if it was cheaper.