| In Ocsigen,
| Quotation.add "xml" (Quotation.ExAst (xml_exp, xml_pat))
| became
| open Camlp4.PreCast ;
| Syntax.Quotation.add "xml" Syntax.Quotation.DynAst.expr_tag
| Parser.xml_exp
| 
| Maybe this can give you a clue ?

Maybe, but I was looking for the equivalent of Quotation.ExStr
rather than Quotation.ExAst, i.e. apply a string transform then
the usual OCaml parser rather than my own parser producing an
AST.

John.