I have argued that this ought to be mandatory ever since Law was first proposed as an ELV back in December 2007, in fact, what you wrote in your post is just about exactly what the wording was in the ELV...
1. Posts and flags around the field
a. Corner posts will be positioned at the outside junction of the goal line and the touch line.
i. If a player is in possession of the ball and touches a corner post he will not be in touch unless he touches the touchline or the ground beyond the touchline.
ii. If the ball is not being carried by a player and it touches the corner post the ball will be deemed to be touch in goal.
In fact, when the ELV's were being trialled in NZ in the Air NZ Cup, that is exactly what was done.
The way they have the corner flags positioned now, it is quite possible for a ball to be grounded on the goal-line, and touching the flag post, while still not actually touching the TiG line. If you have the flag post positioned as in the photo above, it would be impossible for a grounded ball to touch the flag post without also touching the TiG. This makes a lot more sense to me than what is currently happening.