I got to thinking about this and why not just ask our good friend Google on what repositories exist? Inspired by this site, I created a Google query to search for repositores.

Mistral repositores:
intitle:index.of "mistral" +"Packages.gz" -htm -html -php -asp "Last Modified"

Bora repositories:
intitle:index.of "bora" +"Packages.gz" -htm -html -php -asp "Last Modified"

If someone had the knowledge, ambition and time to help out the community they could grab the Packages.gz from the sites, compile a database of applications and compare them to what's stored at http://downloads.maemo.org to find what's missing.

Or, someone could parse the URLs returned into a master repository sources.list. Another idea would be to create a custom search to find a repository hosting a particular application.