Yes, I have the barbell prescription (my favorite of the three books), and read about bunjee suqats. In fact, I use them myself to give me a "tap" reminding me I've hit depth and it's time to go up. Without them, I pretty much always cheat the rep.
There seems to be disagreement about their use though. It's advocated in BBP, but Rip doesn't believe in them, saying you can't train a ROM you aren't reaching.
YouTube
Unfortunately, that principle also applies to deadlifts - he isn't training the muscles used in the bottom section of a squat.
Coach Sullivan also backed this up as Squat Camp, recommending leg presses, so I'm thinking the bunjee-squat/box-squat recommendation might be out of date.
Maybe the 138# of resistance from the machine, plus another 70-100# of me pushing on the chair might be enough to get to an unweighted squat, and from there we can go to dumbbell squats.
-->Adam