Correct me if I'm wrong, but isn't this problem solved by using a symlink? Creating a symlink with the name Default.png and then dynamically generating the file it points to is basically on par with Apple's built in apps.

http://collison.ie/blog/2008/11/dynamic-defaultpng-files-on-...

The article is from 2008 and the SDK may have since disallowed this behavior. I've not tried it myself.