ok seems like yet another premature post! :)
Seems the problem was just impatience on my end, and the UI did indeed show up after 30 seconds or so (compile time I guess?)
I do have a few questions though :)
I’ve managed to hack together a half decent ubershader with SSS built in - I am actually quite surprised how well this did what I expected it to, once I had ironed out the issues with commas in the wrong place, etc. especially given I had never written any MDL before yesterday!
however my shader doesn’t work much at all in realtime mode (it’s good in pathtracing mode) - is that a limitation of externally rolled MDL’s in general currently or more a case of it only supports specific things and I’m breaking those restrictions? (the SSS uses both a diffuse transmission bsdf and a combined surface scattering and volume material as that’s apparently the only way to do it with the built in shading code)
also had some issues getting some attributes to show up in the right groups (I eventually clocked that Create will follow the annotation blocks in the MDL file for this) - you can see here that roughness and metalicity skipped the base group even though they have exactly the same grouping annotation as the ones that are: