After applying the patch, the following function displays a tool-tip
below the cursor in the current buffer, which is something that is not
possible at the moment (without referring to e.g. xwininfo as the OP
explained). 

(defun tooltip-below-point (msg)
  (let* ((win-pos (posn-x-y (posn-at-point)))
         (offset (let ((e (window-inside-absolute-pixel-edges)))
                   (cons (car e) (cadr e))))
         (char-y-offset (cdr (posn-object-width-height (posn-at-point))))
         (abs-pos (cons (+ (car win-pos)
                           (car offset))
                        (+ (cdr win-pos)
                           (cdr offset)
                           char-y-offset)))
         (tooltip-frame-parameters
          `((left . ,(car abs-pos))
            (top . ,(cdr abs-pos)))))
    (tooltip-show msg)))

-ap