Okay, so the previous demo doesn't actually track across eTLD+1's.
Here's another approach:
referer
header to bounce back to this page.EDIT: The referer header only gets passed if navigating within the same domain. But we can just manually pass it via URL param.
Here are some tracking sites. They are from different domains. You can pick one and see what tracking ID it returns.
The returned tracking ID should be consistent, even if you're calling the site from a different origin.