I dunno… sounds like you are making an extrapolation for which you are asking me to spend a bunch of time to do research to support your position which is based on vague references. I thought it sounded like you had some specific details right at your fingertips…. doesn’t sound like it.
Even if I concede that this piece was about the broader definition of ‘soap’ as opposed to ‘liquid soap’ (which it wasn’t), I suspect that there is basically nothing there to find. Were there some wealthy or royalty trying to use something? Perhaps but this certainly wasn’t the norm… just soaking and using hot water was the norm. For some locations, they might have used ashes, sand, vinegar and for some reason, I seem to recall some clever ways they tied horsehair together for scrubbing purposes… and then they likely washed them with hot water.
Here is more, in the book "American Housewife" from the early 1800's there was a admonition NOT to use soap to scrub your good china. If people are not doing something there is no reason to tell them to stop it.
Now your contention that liquid soap is some kind of new invention is, once again, just plain wrong. The first soap was in liquid form, that was why they kept it in jars. It was only later that we learned how to make solid soap.