I don’t think you can mod the maps on Halo 3 like you can on Halo 2, the 360 can be modified to boot 1:1 copies of games but as soon as you modify anything on the disc it breaks the digital signature and the 360 will refuse to load it. Maybe its possible by modifying DLC and resigning it but I’ve not heard anything about that.
There is something about it, I'm not sure if this is what you want but you can check it out here. Just go down the page some.