Okay, so this didn't work out very well :(

Some technical issues with the sound caused the Phone to not hear the backing track at all. it picked up my guitar just fine, but I spent too much time trying to make it work. I'm going to try again tomorrow, using my headset and my computer instead. There's the advantage to the headset that I can just put each of the speakers on each side of the amp (one on the front, one on the back) and place the Mic so it is less than a centimeter from the amp's speaker. If this doesn't work either, well, I'll just have to find another solution, using the stuff I have access to... I should have a bit more time tomorrow.