how would one go about getting access to the source of LessWrong,

I just followed my general protocol for such matters. I googled 'lesswrong source code' and saw it as a publicly accessible project on github. If you fork the repository and commit changes I believe you can issue a 'pull request'.

I would also suggest creating a thread in the discussion section describing your newly implemented feature so that people can express their enthusiasm. If people particularly approve I expect it would be easier to get someone to take a look at your implementation.