I have not been able to find anything reliable. At best, these tricks require access to the other site's code to embed a Flash movie or something similar...

Actually, yes.

You have to add the following javascript statement to your mother page :)

document.domain = 'yoursite.com';

This will effectively allow Javascript access to any of your subdomains. http://msdn.microsoft.com/en-us/library/cc196989(VS.85).aspx