In case you’re wondering, or anyone else willing to give some input while I’m wrapping my head around it, here’s my fork implementing it:
https://github.com/EscapeEntertainmentTeam/UnrealEngine
The fork has the custom shading model work the same way as kusogaki77’s 4.27 branch. Specular attribute doesn’t work yet, however it can be remedied by adjusting specular offset/scale values.