Internal Rules in BNFC for syntax directed translation?

Question

BNFC allows one to use "internal" rules to aid in syntax directed translation. The canonical example they give is for type annotation

internal VarT . Exp ::= "(" Ident ":" Type ");

This gives us a new part of the AST,

Exp = ... | VarT Ident Type

This works great! However, what would be really nice is if it were possible to allow an arbitrary type to be put there as a placeholder for later.

internal VarT . Exp ::= "(" Ident ":" a ")";

And we would generate a parametric datatype

Exp a = ... | VarT Ident a

Then we could fill this in with a type (for a type checker), or a code segment (for code generation).