I posted here a little while back with some code I had used to overrider the stock qty and replace it with a tick or cross.

hope that helps. If it is not quite what you are looking for, I went through the same process as I was trying to achieve a bit of a sophisticated check when checking the type of product options before showing this. - Still work in progress but it might get you started.

http://www.zen-cart.com/forum/showthread.php?t=173383