If you really want to, just work on things that are not related to work and strictly work on your own stuff outside of work hours. Just think about it, they cannot forbid you to edit your .bash_profile or even publish it. It's just hypothetically speaking. Unless you contribute to anything major (which will put you into another position anyways) many people will not care at all for the better or worse. Different story if you start making money on the side. And even then, I once worked at a small corporate where someone from HR worked on the side, I doubt that was ever made official but it was also totally unrelated work.

At the same time, while applying for a job with a running side freelance job, they might offer you to continue that arrangement perhaps with reduced work hours. Just mention your doubts after they make you an offer. (I speak from experience)