A fellow by the name of xbox7887 figured out how to get halo 1 to render in 720p on the original xbox: https://www.youtube.com/watch?v=IJj_XtVKxjc
The only issue is when you scope in with a weapon the game freaks out. You can see it in the video at the 1:35ish mark. If you look it seems to be because the code for the scope still thinks the game render's resolution is 640x480 even though it is now 1280x720. I've tried to do a simple hex find-replace on those resolution numbers but nothing works, and xbox7887 doesn't seem interested and fixing this.
Can anyone here help me out or point me to someone who could? It would be really awesome to have this patch fully working, even if the performance is a little poor on a stock xbox.