I'll reply for Jared since he doesn't seem around. I work on Reason.

It's a single codebase and build system. There's platform-specific code buried in some conditional compilation in the Reprocessing drawing lib he uses: https://github.com/Schmavery/reprocessing

Here's a livestream of the Reprocessing folks making a flappy bird clone (https://github.com/schmavery/flappybird) from scratch: https://www.youtube.com/watch?v=5aD3aPvNpyQ&feature=youtu.be...