The idea of the lm317 is that they are cheap. And wouldn't a 1:1 transformer only regulate voltage, and give different currents for batteries that have different charge levels? Another option would be to use an ATX supply, which can supply many amps at 5 volts. To compensate for the voltage drop, you could add a power transistor on the output. this way you would only need an lm317, a power transistor, and 1 or two resistors per circuit. A nice safety feature and convenience would be a comparator on each charger to isolate the battery. Also, what voltage are the batteries, and what voltage do the commercial chargers charge them with?