Cross Domain Cookie Sync and Auth

Hello, I feel bad dropping in on so many other topics that have appeared around this board, but in light of there obviously being a need by so many I thought I could bring up some tough questions about authenticating cross domain sites and see if I can come up with a solution.

If you login to your main site, you are authenticated for that domain and all subdomains. As I understand it, the problem stems from the fact that you are not on your mainsite's domain anymore and the cookies are not accessible.

Is it possible we could use a solution like this http://flcookie.flensed.com/documentation.php to patch-over the cookies? It seems that is what it was made for. While I'm not a big advocate of flash, and I understand that I would need to add each custom domain to a crosssite.xml in the root of the domain, the fact is most users have flash. It works in 99% of browsers. This seems like it could be a solution to our problems.