It's not Django, but I've built something quite similar for Rails, and have really loved using it. The combination of RDBMS stability and the flexibility of schemaless content is really powerful -- it is (IMHO) the best of both worlds.

Would love to hear people's experience with this kind of setup (no matter what software you're using)...I think it's a very interesting, and useful, direction to go, especially for new projects where you may not want the overhead of multiple storage systems.

https://github.com/ageweke/flex_columns