Other than 'its not the norm' and presuming its mounted securely, what is actually the problem ?

If you research braking systems you can find some 'factory' brake systems far more obscure than that.

So if he had a compressor on the engine, and a shuttle valve on the pedal to allow the compressed air to operate the hydraulics, would that be 'dodgy' too?

Im not considering the installation, more the principles.