Brake Horsepower can not be measured at the wheels

Errr... then why use the rolling road or Dynamometer?

A lot of times a brake dyno destroys the engine (or damages it severely) in the process of measurement

That's because you need to bring the engine to almost stalling point from maximum power, almost instantaneously.This inevitably means the engine and transmission being forced to decelerate to a near stall whilst trying to develop max power (Blimey, just caught myself about to type Mexx Paaaaahhhh ), which will inevitably do naughties to it to due the mechanical stress and (inevitable) overheating. This is rarely done these days as there are less destructive and more accurate methods. As for abuse on a rolling road, well, it's an un-natural operating situation for a car anyway, and overheating while tuning for power is inevitable. Yer pays yer money....

A wheel dyno doesn't factor in fluidic and other efficiency losses between the crankshaft and the wheel.

I think you're being a bit dramatic here; you are measuring the power developed by the engine, and although the camshaft does have direct influence on the power an engine design can produce, it has very little to do with loss at the road wheel. You only measure engine power output at the flywheel, or the driving wheel. Differences between these two measured values are caused by mechanical losses incurred in the gearbox, differential, and other transmission components (including the tyres due to contact patch sizes). Camshafts do not affect this loss. However, internal engine power losses can be caused by bearing friction, pumping efficiency, pumping loss, cam timing and lift, etc. etc. etc. These all result in a figure measured at the flywheel though - which is where we came in.

I suppose it might be possible to build a brake dyno for a wheel, but nobody would be willing to destroy their engine and drivetrain to try it.

... then you have just wiped out the entire dyno based tuning market at a stroke . The driven wheel drums of a dyno are fluidic brakes; one techinique is to run a car at constant road speed on a dyno with a given braking load applied at the wheels. If you time the run, you could measure the temperature rise in the hydraulic brakes to calculate the total energy dissipated during the run. You can then evaluate the instantaneous energy generation, and hence the power.