Yet again as with thing aviation, people will always go out their way to take a simple set of "rules" and make them stupidly complicated.
Charge the hirer, "Brakes off-Brakes on", (Yes there is an element of trust) he should also on the paperwork record, TO-Touchdown for the engine/airframe logs. No problem with recording the Hobbs meter as it will act as a check on those who might make a "mistake" in their timings, as you all know the brain turns to mush after flying and number crunching can be tricky. Simples.
Tacho + 1.25 hobbs -.2 taxying etc etc, what a load of bolleux I'm afraid.