Ah, now I see what you mean.

> On the surface it sounds very doable if each pattern of AST node is generated by a unique grammar production.

This seems to be the key, from my work implementing this in my application. Luckily for me the grammar itself can be somewhat tweaked to help ensure a 1:1 mapping between productions and objects, at least until we release v1, but if someone is having to work with an existing grammar I can see the need for all sorts of hacks.