It's not really that complicated... computers are built to do this sort of thing, and this is a really an easy thing for them.
And it won't really allow you to get top ranks on hards that are worth anything by using HT... because those simply won't be worth anything. There's a difference between taking an extreme map with 7.5 OPS and HT it to 5.6 (which is still a very respectable insane and should count for something) and taking a 3.5 OPS hard and HT it to 2.6 (which is well back in the normal range). Add in the penalties to pp for using HT and the lack of a decent rank, and you've just gained diddly squat for spending 33% more time than if you just played the normal. Plus, the curve can always be made non-linear so that anything HTed in even the moderate insane range barely ranks as an easy. It's just a matter of running the numbers to find something appropriate. There's nothing quite like giving a munchkin hope and then watching them waste hours trying to twink something that's been so curved down that it will never give them anything... it's one of the small joys of game development.
Plus, it's unlikely that a plan that removes scores from consideration will get any ground... that's not far from just unranking the mod, which isn't going to happen. And so we should start by looking for solutions where everything can count, because, ideally, every valid play should be considered for what it is worth when it comes to evaluating pp. And because of that, HT plays should be evaluated at their actual difficulty and against other scores of the same difficulty, and they should not interfere with people that are essentially playing a different song.