If all the primitives (TEXT,MTEXT,INSERT,LINE,LWP,Circle,Mleader,Dimension) are perceived as objects,  
	then you can use this code: 
	 
 
;; Set coordinates of objects (TEXT,MTEXT,INSERT,LINE,LWP,Circle,Mleader,Dimension) Z in 0
(defun zeroz-in-list (lst /)
  (cond
    ;; The list of coordinates-vertex
    ((and (listp lst)
          (= (length lst) 2)
          (numberp (car lst))
          (numberp (cadr lst)))
      ;; if only XY - expanding to XYZ
      (list (car lst) (cadr lst) 0.0)
    )
    ((and (listp lst)
          (= (length lst) 3)
          (numberp (car lst))
          (numberp (cadr lst))
          (numberp (caddr lst)))
     ;; if already XYZ -  just do Z=0.0
     (list (car lst) (cadr lst) 0.0)
    )
    ;; If it's a large list, it's probably nested
    ((listp lst)
     (mapcar 'zeroz-in-list lst)
    )
    (t lst)
  )
)
(defun c:ObjZ0 (/ ss n e el newel)
  (prompt "
Select objects (all types, including polylines, mleader, dimension): ")
  (if (setq ss (ssget))
    (progn
      (setq n 0)
      (while (< n (sslength ss))
        (setq e (ssname ss n)
              el (entget e)
              newel nil
        )
        (foreach pair el
          (cond
            ;; height LWPOLYLINE (code 38)
            ((and (= (car pair) 38) (numberp (cdr pair)))
             (setq newel (cons (cons 38 0.0) newel))
            )
            ;;Point codes (for example, 10, 11, 12...), etc.
            ((and (numberp (car pair))
                  (not (= (car pair) 210))) ; Don't touch the normal
             (if (listp (cdr pair))
                 (setq newel (cons (cons (car pair) (zeroz-in-list (cdr pair))) newel))
                 (setq newel (cons pair newel))
             )
            )
            ;; The rest
            (t (setq newel (cons pair newel)))
          )
        )
        ;; Restoring order and modifying the object
        (entmod (reverse newel))
        (setq n (1+ n))
      )
    )
  )
  (princ)
)
	If the Z coordinate is not displayed in the properties (example, for dimensions, for Mleader), then you need to use the _LIST command.
 
	Don't think of me as a programmer... 
	The code is written using AI.