It's easy to mod the WaW patch because it's not checksummed.
The MW2 patch files are checksummed, and the only way to get passed the headerblocks is to get the .PKG file resigned. And for that, you need access to this:
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(Thanks to Michael for telling me this).
According to unkn0wn (from some PMs we've exchanged), PKG editing is correct, but not in the way we're looking at it. It's not a simple process. He could be leading me astray but i've helped him patch up bugs from his cvar editor so he doesn't have a reason to lie to me.