Lithium batteries have a fantastic run-time, but are expensive and the discharge curve makes it very difficult to predict when they'll die. They'll read 80-90% power for their entire lifespan and then instantly plummit whereas an alkaline or NiMH has a more gradual and predicatable discharge curve.
Considering the cost of disposable lithiums I'd say you're wasting money, but since you need them by Saturday it may be your only option. If I had to take a WAG, I'd say they'll go for 4-6 hours on the 680 running two channels w/ phantom. You should definitely bring extras (even alkalines or NiMH's) and then hotswap if necessary while temporarily using a dvd batt. I believe this will work but try at home first. Definitely not a recommended practice.