Haven't tried it yet, but what about using some form of css replacement of the image or overlay?
Btw, if you are going a more purist route, there are some "internal" switches that would need to be modified as well... Look at the uri when clicking on a Buy Now button. Has action=buy_now... I would almost suggest simply finding a common ground term/phrase to be used throughout.
No the button is not initially product type specific as the code for the button can be found in product_listing, all products, featured products, and new products. An indicator like a product type could be helpful as compared to a category because what if some non-child product were added to the category that is related to the children but not one to be "sponsored"...
So, here's what I did to figure out where the code was to address this: I went to tools->developers tool kit. In the lower left corner I typed in 'buy now'. (No quotes) this brought up a define for: BUTTON_BUY_NOW_ALT (at least in ZC 1.5.4). This provided several files that could be altered to "filter" on the desired criteria in order to display the desired button (though still thinking that might be possible through CSS).