I'm pretty sure I get what you mean, you want to skip the whole step of installing a mod chip, and just hooking up the Xbox HDD to your comptuer and manually plopping the files and folders into the hard drive, correct?
Well, this won't work. Sorry to say, and here's why:
1) In order to play a modded game of Halo 2, you need to have Halo 2 ripped on the Xbox hard drive, while you can store it on there, without a Modded Xbox and a dashboard (such as EvoX) you cannot access this game. It doesn't do you much good to have all the files on there with no way to use them, does it?
2) There's an alternate method to playing Halo 2, in fact, you can just burn all the files with Halo 2 to a DVD, but once again you won't be able to play it without a mod chip.
The Halo 2 mods are a load of fun for a good while, but no matter which method you go about, you NEED to have a mod chip installed. This just cannot be avoided.
But hey, installing one is no problem, really, and most can be turned off easily if you want to use Xbox Live.