How much power would an Arduino Uno board draw if its clock cycles were being maxed out (and it had no external peripherals attached to it)?
The 328 datasheet would be the best source for that kind of information. http://www.atmel.com/dyn/resources/prod_documents/doc8161.pdf
In the electrical section they show this data:
Parameter Condition Min. Typ.(2) Max. Units
ICC
Power Supply Current(1)
Active 1 MHz, VCC = 2V 0.2 0.5 mA
Active 4 MHz, VCC = 3V 1.2 2.5 mA
Active 8 MHz, VCC = 5V 4.1 9 mA
Idle 1 MHz, VCC = 2V 0.03 0.15 mA
Idle 4 MHz, VCC = 3V 0.18 0.7 mA
Idle 8 MHz, VCC = 5V 0.8 2.7 mA
Not sure how one would extrapolate that out to 20Mhz @ 5v, but you can certainly see the trend.
if its clock cycles were being maxed out
This is meaningles in the context of a micro controller, it is always running flat out.
The current of the micro-controller is only part of it. Arduino is a system. So you have to account for the 8u2, power LED, and regulators.
Which if there are no inputs or outputs connected, would be a steady state current.