The support for OpenType fonts has been a great advancement in terms of usability. Unfortunately the
microtype package does not offer font expansion for OpenType fonts in XeTeX. I dug a little deeper into that topic and found that XeTeX doesn't offer appropriate primitives. I would really love to contribute those if I knew how. I cloned the repository of XeTeX from SourceForge, but I can't make any sense of the directory structure. I don't even know where font stuff takes place.
So the questions are: Where do I start with XeTeX development? What are valuable resources to understand the source code? What are the coding standards and how is the code organised?