Could you not do this fairly easily by reusing Untypeast (as suggested in the other topic), which is written in open-recursion style?
@Drup: I believe that the work to add parsetree-relevant information to the typedtree was done by OCamlPro (in particular Tiphaine Turpin, Thomas Gazagnaire, and Fabrice Le Fessant) in the context of the Typerex project – better support for IDEs on top of OCaml.