Even over strictly flat flying areas, the hard deck idea has serious problems:

1. Someone here already pointed out that a pilot's motivation to not land out is partially about points, for sure, but also about the various practical hardships (plus embarrassment) that result from not making it home. The incremental difference in flying behavior would be small at best.

2. The fact that you have scored the poor SOB as landing out due to his "hard deck" altitude, does not place a safe landing beneath him. If the next turnpoint is straight ahead and the best farm field was back over there, one's problems and temptations are not magically resolved by a hard deck rule. The exercise of marginally bad judgement about where to turn back for a safe landing under hard deck rules has a very good chance of having a quite similar consequence as exercising marginally bad judgement under present rules and circumstances. To a significant degree, the problem is moved, but not eliminated.

3. There is no way for a pilot in his cockpit to know whether he has become subject to the rule or not. GPS makes only a crude estimation of altitude. Pressure based altitude works at the home airport where reference pressure is known indirectly by field elevation referencing before takeoff and after landing. The pilot is able to set his altimeter at the home airport. At a remote location late in the day, the pilot will not have a pressure reference available to him and consequently will not know his altitude accurately enough for the proposed purposes. His altimeters are not accurate and furthermore he has no ability to guess how well the scorer's interpolation of local pressure will play out over time and map position. The result will be, that for competitive reasons, he will need to assume that he is not landed out -- likely all the way until exactly the same height at which he would have otherwise committed to a landing. The idea doesn't work.