[benwills]

If I understand correctly, this is the SVN repository for all public WordPress.org plugins:

https://plugins.svn.wordpress.org/

If so, I'd imagine creating a mirror of the registry would start there.

[chuckadams]

There's also https://themes.svn.wordpress.org. I started cloning them on my laptop, then realized I didn't have the free terabytes they likely would take.