I made a post a long time ago about a glitch I found that lets you skip most of the white rabbit quest. Now I finally got around to testing it, and I think I have as much control over it as I can get...
Prereqs for the glitch: Get to the point of the game where white rabbits can start appearing (50 stars and all characters unlocked?), and cannot have caught the green, Luigi rabbit near the painting of Mario.
Doing this glitch involves using either Yoshi, Mario, or Wario. For the sake of explanation let's assume we're using Yoshi. First, you need to catch a specific (yellow, not white) rabbit. Out of the 3 that start out next to the main castle entrance, it's the one closest to the door to the basement (so on your left as you exit the castle, or on your right as you start up the game). After catching that rabbit, switch to Luigi, then immediately (without going through the door back to the main castle foyer) check on the green rabbit by the Mario painting. There is a chance that this rabbit will be white, even though you haven't caught that rabbit for the first time. Catching this white rabbit will result in the game giving you the key to the empty room with the star, regardless of whether you have caught any white rabbits before.
In a TAS, (hopefully) you can manipulate the RNG to make that rabbit white, it's the same as any other RNG roll for the white rabbits as far as I can tell. In a humanoid 150-star run, you would check the rabbit by the Mario painting, and if it isn't white, you would switch back to Yoshi, go through the door to the main castle foyer (to trigger a loading zone), then switch back to Luigi, check the rabbit again, repeat until it is white. For RTA, this could save a lot of time if you get it first try. For TAS it's also a significant time save.
Why I think it works: I believe (through testing on-console alone) that the game only rolls the RNG for potential white rabbits each time you go through a loading zone. Perhaps the aforementioned Yoshi rabbit and Luigi rabbit occupy the same area of memory, or are in the same slot in the "1-7 order of rabbits for XXX character." When you switch to Luigi and go directly to the rabbit by the Mario painting, you don't hit any loading triggers; this is the only rabbit in the game where this is possible.
As I wrote earlier, you can use Mario or Wario in place of Yoshi. For Mario, the specific rabbit you need to catch is the one by Snowman's Land. For Wario, it's the one in the basement. All of this is supported by hours of me running around the castle.
I can make a video of this glitch if anyone wants; I have no trouble replicating it. It'd be a very blurry video of me pointing a webcam at my DS, though.
Not 100% sure how to check what version I'm on, but I got the game right when it came out, so V0 or V1? NTSC.
Sorry for the long post, but hopefully this helps with the 150 TAS if anyone's still working on it, as well as taking out some of the sting from humanoid 150 runs!