@ Slatibartfast, I know you want to adjust the asymmetric clipping amount of the distortion applied to the signal (in a nutshell: differences between positive and negative clipping). And like I said, this is a piece of cake with the proper tools. You can even go way,way beyond the one slider solution you are referring too. Below is a simple example of the scope showing a 100hz Sin wave generated by the Tone generator on which I apply asymmetrical clipping with MWaveShaper.
I know your request is for a native solution, all I say is that it's easy to achieve asymmetrical clipping with (free) 3rd party tools for other people interested in the subject too and looking for an instant solution.