Here's a really crude Google Doc calculator. I've done two tabs, one for a constant braking, the other for 10% duty cycle.

https://docs.google.com/spreadsheet/...nc&usp=sharing

I've assumed 7% grade, 36 kph descent. I didn't include rolling resistance losses since those are pretty small.

The riding the brakes simulation results in 2-3% higher braking losses.

It says nothing about wheel temperatures though. Ideally, you want the wheel temperature to go high then drop back quickly.