In room 315 (outside Little Fungitown hospital)
and in room 381 (Little Fungitown hospital entrance room),
if you set your position to 0 and walk right, there is a hidden warp that sends you to the other room, respectively.
Maybe there are hidden warps in other places. I have found out about this while working on the script.
I'm working hard on the next version of said script. I'm struggling to make the code more reusable and transparent. For things like position, stats, green pipes etc. I'm planning on using lists instead of statically displaying stuff. Should be done with it this week.
List of rooms that have "hidden" warps at position 0
315 <-> 381 Outside Fungitown Hospital <-> Little Fungitown Hospital main room
284 -> 332 Guffawha Ruins and Fungitown Yellow Pipe intermission room -> Guffawha Ruins last room
120 -> 119 Beanbean Outskirts West -> Beanbean Outskirts Northwest
23 -> 0 Stardust Fields room before hammer bros (can't reach later) -> Beta
51 -> 41 Coffee shop -> Bean town east (intact)
55 -> 54 Bean town photo room -> Outside bean castle
91 -> 0 Peach's Castle (cutscene screen) (unreachable) -> Beta
94 -> 0 Stardust Fields (arrival cutscene) (unreachable) -> Beta
101 -> 0 Stardust Fields Highjump/Spinjump tutorial room -> Beta
170 -> 164 On sea -> Piranha Bean room
197 -> 0 Ocean floor room, yellow pipe to mainland -> Beta
217 -> 214 on sea -> on sea
267 -> 247 map screen (unreachable) -> Gwarhar Lagoon green pipe room
312 -> 302 Ship is leaking (cutscene) (unreachable) -> Ship interior, boss's treasure room
334 <-> 334 Joke's End green pipe room leads to itself
382 -> 381 L. Fungitown Hospital Left room -> L. Fungitown Hospital Main room
395 -> 0 Map screen (green pipe selection) (unreachable) -> Beta
396 -> 0 University starbean screen (cutscene) (unreachable) -> Beta
449 -> 0 Mountain lookout screen (unreachable) -> Beta
451 -> 0 Flying to/from Bowser's Castle screen (unreachable) -> Beta
454 -> 0 Flying to/from Bowser's Castle screen (unreachable) -> Beta
481 -> 0 Ship stranded on the ocean floor, cutscene (unreachable) -> Beta
484 -> 0 Ship is debarking, cutscene (unreachable) -> Beta
511 -> 0 Beta -> Beta
512 -> 0 Map screen with Bowser's Castle on it (unreachable) -> Beta
513 -> 0 Level map screen (unreachable) -> Beta
514 -> 0 Level map screen (unreachable) -> Beta
This is looking really useless at the moment. No Bowser's Castle early (yet)
EDIT: I finished testing all the rooms and added them in the list above.
Unfortunately, I found nothing useful.
Please note, rooms 0 and 1 lead to Beta on the English version, but to the opening cutscenes on the Japanese version. The opening cutscene softlocks after you have been through it before, unfortunately.
It looks like there is only one room that you can access where you can attempt to glitch into a Beta loading zone:
197 -> 0 Ocean floor room, yellow pipe to mainland -> Beta
I will try to glitch into that loading zone and if I succeed, I'm going to make a video about it. :)
EDIT: Apparently, teleport glitch doesn't work underwater and you can't really use any other glitches to reach that place. Well, except if you wrap around the entire room maybe, but that would take like 4 hours.