Hand brake mechanism sounds like a good idea, however, in my experience there are lots of hirers that will work that to set the brake to a position it won't tick over. The oil pressure switch is pretty fool proof for charging as it is difficult to tamper with or circumvent. Old fashioned tacho or flight switch even better as you promote proper warm up and taxi technique to reduce wear and tear on the aircraft. Although I have seen airswitch tampered with to slow metering. VDO/HOBBS inevitably pushes users to 'rush' into the air and possible increase wear on things. I was always in favor of Airswitch usage for charging and a flat taxi fee for consumables on the ground. Operators don't like this though as they get an extra .2-.3 on the ground where the maintenance is technically not ticking over when using VDO/HOBBS and therefore it covers misuse theoretically.
I've also seen a few Ag ops refueling/refilling with engine running while the pilot takes a break, more than a few times.