Faking an active region

[Deniz Dogan <deniz@dogan.se>]

I am writing a minor mode in which I want to remap `undo' to ALWAYS act 
as if a specific region was active and transient-mark-mode was on.

So how would I go about "faking" this active region in Emacs Lisp?  This 
is what I have so far:

(define-key map [remap undo] 'nima-undo-undo)

(defun nima-undo-undo ()
   (interactive)
   (let ((transient-mark-mode t))
     (push-mark nima-prompt-end t t) ;; a marker
     (goto-char (point-max))
     (undo)))

This doesn't work, and I'm not surprised because I feel like I'm just 
throwing everything I can find at this problem right now.

Thanks in advance,
Deniz