To me, resting the finger 'on' the trigger guard means it's across the trigger and too close during a run-and-gun situation. EXAMPLE: MY finger WON'T REACH the front of the trigger guard, so if it's across the trigger it's NOT a safe practice.
The RO has to make the determination necessary to maintain a safe shooting environment. I would fully expect the RO to either 'warn' or more likely DQ a shooter with this practice. The rules do give the RO decision making capability. The shooter can file an objection if he/she disagrees--good luck.
Again the rules say, "Finger outside the trigger guard," it doesn't specify where it should be outside of that. So as long as the finger is outside the guard the SO/RO shouldn't be DQing a person if they don't embrace the finger on the frame.
If an RO/SO feels that it isn't good enough, he should contact the IDPA, I just got an email that they are going to relook at the rule book via "Tiger Teams."