This was the very thing I recently groused about not being able to find. Indeed, it contains rather interesting guidance. Concerning the use of the 85mm Soft -
"... The picture taken with the Rear Converter combined is more uniform, better in quality and has a greater soft focus degree than that taken with the soft-focus lens only. Combining the Rear Converter varies the focal length and speed..."
The Rear Converters are the Pentax 1.4x and 2x tele-converters. Four of them are listed in a table found just below the quoted guidance.
I'm not sure what "a greater soft focus degree" means. After having dug into the topic and trying a 1.4x teleconverter with the Pentax Soft perhaps what they were trying to say is that the degree of softness across the field of view is more evenly distributed than when the lens is used without a teleconverter. That, at least, is what I found.
Have a look at the following image. The softness of the scene at f/4.8 is quite nicely distributed. When used without a teleconverter, the scene shows a strong sharp to soft transition zone. It's something that I called a "resolution hole" in the series of nine blog entries I posted on the topic of soft focus Pictorialism starting here.
Using the Pentax 85mm f/2.2 with a teleconverter seems to do the trick. It behaves rather nicely and reminds me a lot of the results that can be achieved using a Wollensak Verito on large format film.
Pentax 85mm f2.2 Soft at f4.8
+ Sigma 1.4x Extender ~ USM
RawTherapee Image Processing:
+ 100 Micro Contrast
+ Red filter Channel Mix
+ Noise Reduction -1