Where can I propose open source project collaboration software?

I suggest you to use http://kickstarter.com or a similar website to make a proposal and gather some funding. This will enable you to more easily manage your project and set achievable goals, even hiring programmers if you need to.

In second place, you should start the project and host it somewhere public (let's say github) and try to spread it via twitter, freenode IRC network and that kind of medium.

Finally, if your project's suitable, you might want to try approaching relevant bloggers on the field to get some publicity. This will serve a double purpose since if they think you're going nowhere that might be meaningful. So, use this as a interest meter also - but keep in mind it could be just due to an early stage of the project.