MidNite Solar DIY Series

The high voltage limit is usually determined by the filter capacitors.   63V is a common capacitor value.   
If the capacitor voltage went to the next higher voltage rating, then the capacitance would be drastically reduced.

A custom capacitor could also be used.   But the NiFe battery market is not high enough to warrant that expense... BUT you can certainly put two "chargers" in series on each half of the NiFe battery bank.  We have done this many times with MPPT controllers.

 Wouldn't want to do that with an inverter though because of the uneven loads on each half which would more than likely ruin the batteries.

But charging each half of the bank is fine as the charge voltage is the same on each half.