that all
yes but simple, as vraycar paint
ah, I didn’t notice it’s a feature request, sorry 
This reminds me of the absolutely brutally fantastic feature of Unreal Engine shading system. A Master-material.
Basically, you create a simplified instance of your shader with any parameters exposed you want. No coding required.
So for example a car shader, you would build the complex network of full shader using regular CoronaMTL (a “Master shader” ), and then create a shader-instance from it that would expose glossiness and color for example. Anytime your would create another color of car shader, you would use your master-instance, which even 3 weeks old monkey would be able to, using just 2 clicks. People could share those.
Of course, another benefit is the propagation from master into instances but that’s other topic.
A full OSL support in Corona would do this in more complex way. But master/instance shader is fantastic thing to create as many specialized ‘fast-to-use’ shaders without having to hard-code dedicated shader for everything (car-paint, skin-shader, sand-shader,etc…).
Material instancing in unreal is useful mainly cause no recompiling is needed for changes made to instances. What you want here is more equivalent to node grouping in material/node editor, similar to how node editor in blender works for example, where you group nodes and create input/output parameters. I guess that however is a feature that max would have to support in it’s slate material editor in the first place.
You’re right, a simple node group would make it, but I would still want a simplified export of it. Such gui overlay could be used in simple mat editor for very fast tweaking without any access to underlying shader tree.
It would allow me to make set of quick Autodesk style arch shaders.
True, slate isn’t even a new thing by now