Cell Voltage Relaying in a Battery Management System
I'd consider using switched-capacitor isolation amplifier like (but not) the LTC1043. Basically, analogue switches apply a capacitor across the voltage to be measured, the cap charges up in a milli second or so then the analogue switches return the capacitor to ground so that voltage measured can be determined by a regular ground referenced amplifier.

The LTC1043 doen't have sufficient input voltage range for your application but something like it could easily work.