I think the question in the OP has been answered. I believe that cliveh's point was that it is permissible to modify a developer for certain specific reasons. In this case converting a developer to a monobath. (Whereas just mixing two developers together in the hope that this will result in better results is probably misguided.)
The silver content or additions to the emulsion should not affect how a monobath works. Modern films may require a slightly different formulation for the bath but Haist gives instructions on how to do this. In a way modern emulsions, being pre-hardened, may remedy one of the classic problems of the bath causing unwanted softening of the emulsion. For anyone interested give them a try but a bit of experimentation may be required.