Wait, do you mean "it wouldn't carry over on the console" as in like to your 360 or PS3? Thats almost what it sounds like you mean. If so, that is a complete no no!!!
Otherwise forget the fact that I even posted this, and I'll probably heavily edit it once if that's your response.
Why?
It doesn't say it anywhere in the rules on this site about Xbox 360.
It is not illegal since the Xbox 360 has not been modified and the only thing that gets modified is save data, which is transferred using USBs and that was made possible when Microsoft allowed USBs to be used on the Xbox 360.
Bioware actually has a tutorial on editing saves for Xbox 360 and PC with all the downloads required for Xbox 360 http://social.bioware.com/forum/1/topic/307/index/6749366/1 on their official site. So that even proves that it doesn't break anything. Even Xbox Live's terms of use states "...affecting other users' gaming experience" and since FoNV is a single player game it doesn't affect that.