Thanks, Jack and Sven, for the helpful discussion on this.
My impressions are that (a) implementing the Eviews-type thing
would not be very difficult but would be of doubtful value, while
(b) implementing the full-dress version would be a major task,
which, if we reckon it's worthwhile, would have to take its place
in the TODO queue along with the user-defined Kalman filter,
Blundell-Bond and SVARs.
So I'm inclined to put this on the back burner for now.
Allin.