Is there anything (in Scala, Kotlin, Java, whatever) that fullfills your very obscure requirements? I haven’t seen a single attempt at putting field indexes into a type system.
Yeah, there is at least Haskell superrecord library - SuperRecord: Anonymous Records for Haskell that morphs morphs labels to indices. SML#, according to stackoverflow, uses implicits to pass indexes for polymorphic records - Why doesn't OCaml support record subtyping? - Stack Overflow
Also, so far all discussed implementations of structural types for Haskell in https://github.com/ghc-proposals/ghc-proposals/pull/180 are index-based, not hashmap-based.
However, all of the above are for row types without subtyping, I’m not aware of any implementation of index-based access for subtyped structural types.