I have not spent any time on monobath, it is something on my long list of todos! I was planning to try it when I finally ran out of 55 and 665 (I haven't yet). Maybe I will try it this winter. But I think David Goldfarb has done it- you could ask him. But look, type 55 was almost certainly just panatomic x developed in a monobath. We've had various discussion on this here and it's true we don't have the proof... but it is very, very likely.
Look at this thread: (there was a url link here which no longer exists)
Bottom line is that if you are dedicated to one film type shot at one speed, then a monobath is probably the ticket. They fell out of favour when people started shooting a bunch of different films at different speeds. At that point there would have been the need to optimize the monobath for each... so that was the beginning of the era of the general purpose developer.
Other than monobath, I suppose that rodinal may be another interesting thing to try with pan x.