Relatively easy if we can find the memory location that's responsible for going to the Boss Level. According to my notes, we already know how to prevent the Crystal Ball to appear, so that's one thing off the list.
Current level number is stored at $401 in RAM, one-based, which means level 1 corresponds to the value 01. I did a test and beat level 99 in the standard way, and through the door. When you take the door, the level number goes from 99 to 100. Level 100 is simply displayed as A0 at the top of the level. When you don't take the door, the Boss Level loads but the variable stays at 99. This means there's another flag somewhere that tells the game "load the Boss Level", and it's not $401. We have to find that. Most likely, the Secret Door changes this flag.