High Voltage DC - Shunt Resistor, Voltage Regulator Question
Allowed Voutmax = 100 VDC.
  Available Vin is 60-100 VDC
  Imax &lt;= 5A
  Want to limit Vout max to 100V so Vout is then 60-100 V.  


A fuller description of the load and the source variation characteristics is necessary to give a good answer. 

If you can ALWAYS tolerate a 10V drop then a 50 W zener diode (or bank of parallel lower wattage diodes) always in circuit would be the easiest and possibly overall lowest cost solution. 

The "problem" is that doing this with a linear regulator will dissipate up to
P = 5A x 10V = 50 Watts. If you are prepared to dissipate this in a semiconductor (probably some or more FETS) then  a simple circuit that meets you spec is possible BUT it may not meet you real (so far unspecified) needs.

Without knowing what value you place on this solution its hard to know what approach is suitable. In both the cases below for one off the design and build costs would far exceed the material cost. 

eg Using 5 x 12V 1N2976B 10 Watt zener diodes in parallel mounted on a suitable heatsink and switched into circuit by a MOSFET  when Vin exceeds 100V would cost about $US25 for the zener diodes plus the cost of the heatsink and glue electronics. 

A PWM switched series resistor - perhaps a length of Nichrome (or similar) wire and a smoothing output capacitor and basic PWM controller would work. 

An off the shelf ebay switching regulator (buck converter) may be the cheapest and fastest solution.

Q1 What is your budget?
Q2 How fast does Vin rise to beyond or below 100V?
Q3 Can you tolerate 10V drop at all times eg if Vin is 60V?
Q4 What output ripple voltage can you tolerate.
Q5 One off or how many?