This should work for you... not it's original intent, but effective nonetheless.
It reports in feet, but it's actually drawing units.
Code:
;; returns horizontal distance regardless of z values - LPS 2008
(defun c:di (/ pt1 pt2 pt1-flat pt2-flat hd hdtxt)
(setq pt1 (getpoint "HorDistance Pick first point: ")
pt2 (getpoint "Pick end point: " pt1)
pt1-flat (list (car pt1) (cadr pt1))
pt2-flat (list (car pt2) (cadr pt2))
hd (distance pt1-flat pt2-flat)
hdtxt (strcat "\nHorizontal distance = " (rtos hd 2 2) "'");(getvar "luprec")
);setq
(PRINC)
(PROMPT hdtxt)
(princ)
);defun