You can use action replay saves, but if you have the patch...and the other person does too...then Im not sure if you can have silent weapons or what not. Also, if you wanna be cheap, you can take advantage of that walk through walls, even if your not host. What you do is wait for the countdown, then push the button that you assigned Ghost to, now you have like 2 or 3 seconds to move up or down or wherever you want to go. So if you wanna be cheap, move down a little to were your chest is at ground level. If your chest/head is at ground level, NOBODY can see you! So you can walk around shooting people in their crotch or legs or wherever and crack up!
I hope my tutorial thingy helped for the .umd hack. If you want to IM me on AIM, my screen name is Digital Marine68.