How can I run a 6.5V-100V power meter on 5V?
for 5V operation you'll need to loose the diode D1 and THE voltage regulator formed by 
R3 R4 R5 R7 Z2 C13 Q1 
and feed the apropriate voltage (probably 5v) to the Q1 terminal closest to E4
probably best to measure that first.

you can get some reverse polarity protectopn by putting a reversed diode in parallel with the input. but if you have a 100A 5V supply that won't work well.