To answer to myself:

EXTEND Gram
>
> GLOBAL: sig_item;
>
> sig_item:  [[
>     "module"; "type"; i = a_UIDENT; "="; mt = module_type; "with"; id =
> LIDENT ->
>         <:sig_item< module type $uid:i ^ "_" ^ id$ = $mt$; >>
>     ]];
> END
>

should have been: str_item instead of sig_item + any keyword different from
"with" after the signature definition.

- Thomas