Don't do this in Django. Just don't.

This should be done with a database proxy server. With MySQL, you can use MySQL Proxy and PostgreSQL has pgpool-II.

Yes, you could hack the Django code to solve your problem. Why do that when there is a very simple and tested product that already does that transparently? For me, supporting multiple databases in a web framework is akin to writing an HTTP server into your web framework. Maybe you should do it for testing purposes, but not for production.