Related tags were one of the "maybes" slated for API V2.0.
It got implemented today.
Note that our definition of related is pretty much "appears on the same questions as", there's no real secret sauce there.
Instead of linking to a .tar.gz archive, don't you think it's more practical to link to the raw file instead so it can be installed immediately?
Click here to install the script
It seems more logical to me.
Also, there is a small problem that you can add tags to your search query multiple times.
If a tag is already in the search query, it should not be ...
This was added in the last deploy.
Originally I was planning for consumers to maintain the site cache based on host names, but I suppose it's better to have the "query with this" parameter readily available regardless of how a site is discovered.