From my exploration of the DS shader environment, there is a sort of material room in DS - Shader Mixer - but it is not the same as Poser's material room at all. Node connections use an over-ride mechanism, not multiplication, which makes the process unnecessarily complicated since every time you want to multiply you have to insert a discreet node for that purpose. And it's not like Shader Mixer has heaps of available real estate. Documentation for Shader Mixer is sketchy, so it is not the same environment at all.
There is far more documentation for rsl scripting, but writing shaders for 3Delight is not for the faint of heart: you need to have a working knowledge of C++ since the syntax is similar.

All in all, not for most user's consumption.

Quote - To the best of my knowledge, Poser's Firefly is also based on a Renderman-compliant engine though it doesn't allow you to use hand coded shaders.

Not true: I've been writing shaders in Python for the Poser material room for some time now. BagginsBill has developed a powerful shader-writing tool called Matmatic for Python and Poser. Using this tool, you can write some incredibly accurate shaders. By 'accurate' I mean: using mathematical representations of physical properties of materials. I'm surprised more developers haven't caught on just how incredibly useful this tool really is. Python is infinitely easier to learn than C++ and so one can get up to speed very quickly.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks