Have you looked at buying a low voltage disconnect like one of these?
Elite Power Solutions
I thought about buying one, but I'm most likely going to end up with GBS batteries and Elite power solutions makes a BMS that is designed for those cells and that seems like where someone might find a good market, if you can build a BMS for the A123 system batteries that fit with the batteries.
I've had a few people tell me that if you have a low voltage cut out and a charger that cuts off at the right high voltage that you don't need a BMS but that at that point wiring it up so you can charge each cell to bring them up to a balanced charge should work just fine, the people who have suggested a set up like this of course said that if you check the full or empty voltage of each cell after a year or two that you should find that they don't drift out of balance.
I of course like the idea of a BMS that can talk to the charger and cut out at low voltage.