name is guaranteed to be constant. So is site_url*.
Use one of them.
/users/{id}/associated is not vectorized (as a part of stackauth and not api.* it doesn't follow the "all {id}'s are vectors" rule) and thus you should be able to keep track of that information yourself easily.
*There might be some goofiness with area51 created sites in this regard, we'll keep you posted when the details shake out.
Update.
The proper way** to lookup site data is to key off of the current domain (http://stackoverflow.com, for instance, site_url basically) and resolve old/cached entries using aliases.
Its probably going to be months from now before any site returns anything in aliases - whenever WebApps becomes... not WebApps, assuming it ever does - buts its spec'd.
**As opposed to synthesizing some "key" and trying to hold it constant in the face of changing site data. Pro-tip, this is how the SO engine works!