Send them your portfolio and tell them why and how you can be very helpful to them. Easy ?
Of course this causes a class divide between:
A) People who are paid to contribute to open source (either as part of their job or in some cases their whole job) and therefor have many commits.
B) People who write a little open source code on the weekends.
C) People who already work 60-80 hour weeks on proprietary/in house software and have other commitments for the rest of their time.
Also the "you need to contribute to open source to get a new job" thing has a perverse incentive for companies. If they know that having you contributing to open source makes it easier for you to either leave or demand a higher salary (because it is easier for you to get hired elsewhere). It is then in their interests to stop you doing it whenever possible.