[Greasemonkey] Any way to do stuff before the DOM is fully loaded?

[Luiz-Otavio Zorzella]

I want to be able to override a JS function right after it is loaded
from a .js file. Converselly, I could get away with setting a variable
before some JS within BODY is executed.

But, apparently, I can only do GM stuff after the full DOM is loaded.

The situation in question is this: AtomFilms (.com) has a detection
engine that looks for a plugin named "Windows Media Player Plugin"
(analogously for Quicktime), and, if not found, will not let you watch
these formats, even though I can play them in my system with some other
helper application.

If GM cannot help me, I'll have to try to create a bogus plugin just to
pass the JS test.

Any ideas will be welcome.

Zorzella