That worked
It was the civilDocument part I think I was missing.
I think I tried to access the CorridorCollection (and AssemblyCollection) from the standard AutoCAD Document object and obviously got nowhere.
Now knowing this, I do find it strange that i could then directly access an assembly object using civDoc.AssemblyCollection[nameOfAssembly] but once i had it, that object appears to have no way to see and access the subassemblies attached to it? As Jeff_M says above, the only way i seemed to be able to get to the parameters of a subassembly was going through the corridor hierarchy and extracting an AppliedSubassmbly object. Is this true, or have i missed something else in the documentation?