Nice idea. Might work. However, it looks like you can't get the actual Steam integration module unless they accept your game. They claim it's an easy add-in to your code.
Check the FAQ at http://www.steampowered.com/steamworks/FAQ.php
Sounds like you need a demo first, and if they like it, they'll provide you the drop-in C++ module to make it Steam-Powered.
I did look through their catalog, and there aren't any Mac-exclusive titles for sale right now, but then again, most games that are on the Mac Steam are from major label companies.
I think the major factor we need to consider is size & scope of the games. Developing for iOS, I know I tend to think of small memory footprint, highly optimized, casual games. Once we (as a community) can get our minds around big, giant, 200+ MB hardcore games, I think the chances of cocos2D titles on Steam are pretty good.
Now you've really got me thinking...