Thread-bump: I think I should let y'all know that I've found an alternative:
JScript code:
/* This is code executed by Plus! */
var document = WndInternet.Browser_GetInterface("BrowserCtrl").document;
var window = document.parentWindow; // This is a window object as seen by HTML/JavaScript
window.alert("Hello world!"); // actually works!
This works even though IE is in compatibility mode. I think it's a standard, so it's unlikely to be dropped in future versions of IE; but I might be wrong about that, dunno.
/ 
