MAH can probably memorize anything. I guess once you get to that stage memorizing the ENTIRE busoni concerto is nothing. Even though it is not my favorite concerto - I was AMAZED. Also, I've heard him play the Alkan concerto on the radio - which is pretty stunning - and imagined that he must have memorized it to play it so well. (Alkan only wrote one didn't he?)
Even though my memory is much worse (when i don't exercise every day at least an hour) - i find that repetition still helps. And, giving oneself little form cues or make up stories to go with the piece. Even 20th century pieces can have some similarities of repetition (even if not exact). You have to look for similar intervals or sounds or patterns on the keyboard. Everyone here knows that -but it's very different when you combine nerves and a whole slew of notes that don't make harmonic sense on the first hearing. I'd say Barber can make me terribly nervous although that type of music really increases a musicians awareness of where the notes are on the piano - and you start being able to hear when you hit a wrong note - or where you should be on the keyboard to get the sounds you want.
Surprisingly, I memorized (which is hard for some) the entire Bach English Suite III several years ago. Many years ago, I thought one of my peers who memorized the Italian Concerto was genius. She probably still is genius, to me, but insanely dexterous too.