The advice I give in regards to where to look and how to go about improving your chances is exactly that. People who post vacancies on HN & GitHub are looking for hackers, not code-monkeys.
http://rawsyntax.com/post/6249655944/how-to-get-a-programmin...
Finding the opportunity is the hardest part and cover letters aren't as relevant as what you may think. I'm sure your post is great for interview advice, mine was aimed at helping people get those interviews in the first place.
My experience tells me that cover letter relevancy is subjective. Some managers swear they need to read a good one, and others flip right to the resume.
I know this was useful from my experience since the interviewers have mentioned that cover letter was very useful or they brought up details from it.