It's taken a week longer than I had hoped, but the private alpha is now ready. As mentioned previously, there's a bunch of features not yet implemented and a handful of known bugs, but I've been able to test the installation package on another machine and it seems to work fine. There's a dummy game installer included which uses Microsoft's Click-Once installation to ensure you have all the requirements (dotnet 2.0, XNA Framework 3.0) and automatically downloads them if you don't. All this is covered in the readme.
If you'd like to participate, please email clarvalonATliveDOTcoDOTuk and I'll email you the link to the file (rapidshare). Please post any feedback or issues on this blog.