A v6 version should be possible to build on top of @nathanletwory s pbr plugIn Rhino PBR-Style material
For the first public version of RhinoBridge I wanted to just built on already existing functioniality, which is why I chose v7 with its build in PBR support. Although quixel uses different names for their texture slots than Rhino, so probably some things are lost in translation there https://developer.rhino3d.com/wip/api/RhinoCommon/html/T_Rhino_Render_RenderMaterial_StandardChildSlots.htm
Rhino 5 would be out of my abilities, also I don’t own a copy of it so it’s very unlikely to ever happen 