This is purely my opinion but here's the conclusion I've come to after reading posts on hill-hold, creep behavior, and the stories of problems with people pressing both pedals at the same time. Tesla should modify the existing behavior (I'm assuming this could all be done via software) so that:
1) Pressing brake disengages any accelerator input. You get the warning beep AND there is no forward propulsion even if you also press accelerator.
2) Hill-hold doesn't need to be a new feature - just modify Creep to include a hill-hold behavior. If you have Creep turned OFF then the car rolls backwards when no pedals are pressed. (My preferred daily mode of operation). If Creep is turned ON then not only does it slowly creep forward when no pedals are pressed, but it ALSO will not roll in a direction counter to what the 'transmission' (R or D) setting is.
If you're really in a situation where you're concerned about rolling in the wrong direction, turn on Creep and don't worry anymore because the car absolutely, positively won't move in a direction you don't want it to.
Note: When I use the term 'hill hold' I'm not really concerned about the actual technical implementation of how this is accomplished. It can be using brake, motor, or both. I just don't want it to roll in a direction I'm not telling it to move.
Having Creep implemented this way also removes any need that I can think of for why you'd want the accelerator and brake to be active at the same time.