Using document.title='new title'; in the modal dialog page does work, 
after all -- I was confused because I was using Visual Studio ASP.NET and 
trying to register the script using the VB codebehind, rather than writing 
it directly into the HTML.  But because I didn't have a FORM tag in the 
page, ASP.NET's register script block function doesn't work (and doesn't 
issue a warning, either!)

Thanks to everyone who had suggestions!

-Van