According to Jeff's comment on this answer, users already get a notification that they should associate their user accounts when logging on to the other site:
new accounts get a topbar notification reminding them to associate, with a link to the associate page. Short of going over to their house and clicking the buttons for them, not sure what else to do.
So, the next time the user revisits the original question, they will be automatically redirected to the site that is its new home, and prompted to associate their accounts. If, somehow, they fail to be redirected to the new site, they will see a banner at the bottom of their question indicating that it has been migrated elsewhere, with a link to that question on the new site. From your example:
migrated to serverfault.com by Mitch Wheat, Bill the Lizard♦ yesterday
This question belongs on our site for system administrators and desktop support professionals.
Once they've associated their account on the new site, they will automatically regain ownership of their question, just as if they'd asked it there originally.
I'm not really sure what else we can do other than automatically creating an account for someone on the target site whenever one of their questions gets migrated, and I'm just not convinced that this is necessary. How often do you really see this being a problem? From the linked question, it looks like you're simply assuming that it might be an issue, rather than actually having seen it become a stumbling block for new users.