Even if you don't own the code you've written, you can ask your employer in advance for permission to show your code to they potential future employer. If that helps, you can make the future employer agree that they must not actually use the code you show them for commercial purposes, they may only use it to evaulate your application, and must delete it later. Maybe the past employer is more likely to give you permission under that condition. Similarly, you may want to show only some selected short but interesting parts of the code, not the whole huge thing you wrote. Of course, all of this depends on your past employer: if they're an asshole and don't care at all about you once you've left, you can't really do anything.
