> > use that with (interactive `(,@(DTRT))).
> A.k.a (interactive (DTRT)), right?

;-) Yup/oops.

I was thinking of the more general case of
region/buffer-limit args, plus additional args.

(defun foo (beg end other)
  (interactive `(,@(dtrt) ,(+ 2 7)))
  ...)

or

(defun foo (begin end other)
  (interactive (append (dtrt) (list (+ 2 7))))
  ...)