Okay, so update on this... I've been doing assisted pullups with the Pullup Revolution thing, and even though I can't get a pullup from a dead hang yet, I *am* now able to get a lot closer. I can get from a deadhang to maybe 60 degrees. I need 90, of course, and it took me several months to get from 0 to 60, but it's definitely a start.
I've been using all three bands, which say they offer 60 pounds of resistance each, but since I weigh less than 180 (but more than 120) I'm not sure how accurate that is. I can do about 8 pullups with full resistance (starting as high as I can--so pretty much max resistance, max bands). I am thinking of dropping down to just 2 bands at some point.