Somewhat the same - even without target cooperation, NASA can either
look up, or knows many of the terms we have to derive or constrain.

I don't believe that's true, at least to a large degree. Note that Henry,
in his original reply, said that the solution also involves "measuring" the
masses and orbits of the planetary objects involved. Remember G is only known
quite inaccurately (10^-5 or so, IIRC), GM for many bodies is known more
accurately (in large part due to such navigational exercises), and there are
some high-order effects that are difficult to characterize analytically. It
will be an "interesting" exercise to navigate Dawn, in particular.

Jan