That is difficult. How about-

• Get those who have seen the proprietary stuff to recommend you on LinkedIn; target the recommendations to what you can't show. Coworkers/managers who liked your work may endorse/sign anything you present so don't be shy about paraphrasing what you want in a recommendation (if you're sure it will be well received).
• Write white-paper-ish treatments of what you've done; discuss the problem space, approaches, related tools, and technologies; making sure not to cross disclosure lines. This will not show code but demonstrate your understanding and ability to self-manage. Brevity and clarity make this kind of tech reading enjoyable for the widest audience.