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
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