You can get isolated DC-DC converters very cheaply, which will output 5v @ about 200mA.
If I were you, I would design a small board that takes in 5v, isolates the control and output voltage via isolated converter, has a small micro and some control circuitry to set the output voltage. You would tell the micro what voltage to output via serial, using an opto to cross the isolation barrier.
Make up 14 of these boards, stack them in series, and use addressing to set the voltage of each 'cell' as desired. If you don't need to sink current (simulating charging), it's a very simple and elegant solution!