Simple cross domain tracking

I hear of some really complicated schemes from time to time to track users across multiple domains that belong to a single site. While I'm sure they mostly work it seems like there's a simple way to do this that I assume many people are already using but is probably too boring to comment on. So, let's be boring for a moment.

Let us say you own eggs.com, bacon.com, and coffee.ca. When a user visits eggs.com he is assigned a unique tracking token in the eggs.com cookie (we'll call it [tracking-token-eggs]). At some point after that token is assigned, include it in the page requests to //bacon.com/tracking.gif?token=[tracking-token-eggs]&domain=eggs.com, and //coffee.ca/tracking.gif?token=[tracking-token-eggs]&domain=eggs.com. (Create the same setup for visitors to bacon.com and coffee.ca).

If the browser already has a token stored in the bacon.com or coffee.ca cookies you will now have a request that includes both domains and both tokens; both domains are in the url, one token is in the url and the other token is in the cookie of the request. The first domain is also in the referrer/referer. This works even if 3rd party cookies are blocked (at least in the browsers I've tried). Now you can store this request in a database table or just a log file.

If you want to do something slightly more complicated that involves javascript you can alter the technique to use iframes instead of gifs. Just don't try to create or store any new tokens in the iframe from the foreign domain because this is when techniques fail.

[Edit: I should add that this is a technique for when you have half a dozen domains or so. Not for hundreds of domains.]