P.S. Of course, I'm curious about your other methods but I imagine those are your "secret sauce."
Have to leave something to bill for..
Also, as far as use cases, take a company with a blog that's tightly related to cross-promoting their product but has its own domain name. Or a company that uses ccTLDs for their localized sites (example.com.au, example.fr, example.de). Or companies that have recently merged (and merged their product lines and, in theory, marketing efforts) but have different CMSes. In these cases, navigation across domains would be expected, but the moment that happens, you lose track of the human lead.
Of course you could (re)architect around the easier cases, using example.com/thebigblogname for the blog in the first case and paths for localization in the second case (/au, /fr). Some would say that you don't care about the multilingual user who would become known on a Spanish-language site and then organically visit an Engligh-language site. But overall, it's not a crazy situation.
This sounds like a better job for a tracking pixel or a 1x1 iframe large enough to read and write a cookie.