Despite having written tens of thousands of lines of open source code, I have yet to have an interviewer who has looked at that code and asked me about it.
As an interviewer, Open Source contribution is one of the first thing I look for about a candidate.
Just having committed to an open source project would put you way ahead of most of the people we interview.
If you do this, please take into account the time they wrote the code too - they might have improved their skills significantly since then.