Not sure if this is the right place as I don't know how much of a breakthrough this is so mods please let me know if this should be moved elsewhere. There's been multiple posts asking about how to remove the banned Pokemon restrictions on Battle Maison in X/Y/ORAS (Some people are even offering bitcoin incentives to have this figured out). I've spent pretty much the entire day working on this/trying to figure this out but for the life of me could not, so to have this day not go to waste I'd like to share some of the progress/things I found out and discovered along the way. Hopefully someone out there can pick this project up and finish working on it.
So how do you remove the Battle Maison restrictions? My conclusion, after a lot of experimenting, is that you have to edit the DllBattlePartySelect.cro file. Here are my reasons:
1) After messing with that .CRO file, I rebuilt romfs using PK3DS, loaded the patch using Hans, and my game was running completely fine up until the point where the Battle Maison lady asks me to select Pokemon. The game freezes at a black screen and I'm forced to power off.
2) I messed with DllBattlePartySelect.cro by reading it through a Hex editor. Call me crazy, call this a conspiracy theory, but there are 31 instances of the Hex-value sequence "FE FF EB" in that file, and there are exactly 31 Pokemon banned in Battle Maison.
Now I know it's been said before that CRO files can't be edited, and if they do then the game just crashes, but after some research I came across this thread and heard people saying that CRO editing works with Luma3DS (I use Gateway3DS for launching Hans using homebrew). So I spent time setting up Luma and between the CRO resigner and Luma I couldn't get anything to work lol. After patching static.crr with cro_tool.exe the game wouldn't boot so I used the old static.crr, and patching the romfs into a .cia file for Luma3DS didn't work either...
So in short, editing DllBattlePartySelect.cro by modifying the 31 iterations of the "FE FF EB" hex values is my best guess at figuring out how to remove Battle Maison restriction (I am using Alpha Sapphire, sorry if that becomes relevant). The million-dollar question is figuring out how to edit CRO files using a Hex Editor without having the game crash. Maybe Kaphotics or SciresM would know how to do this. I know there are some CRO editing capabilities that Pk3DS has, but still no way to edit that golden DllBattlePartySelect.cro file.
Edit 9/24: Solved for ORAS. Still need to find the garc location for X/Y (if anyone really cares). As well as for SuMo's Battle Tree. 80% sure this will be the same for Ultra Sun and Moon, but it would be naive for me to say that about a game that hasn't even been released yet.
EDIT 9/25: Confirmed working for Sun and Moon. tl;dr: GARC location for ORAS is a/1/7/0, for SuMo it is a/1/3/7. Replace the bytes quoted by Kaphotics with 0's and you're good to go!
EDIT 9/26: You can now remove Soul Dew clause in Gen 6 games, rendering the banlist completely lifted! The only type of Pokemon to still be banned in Battle Maison is one whose total EVs exceed 510 (this is allowed in SM, don't ask). (REDACTED BUT EVENTUALLY SOLVED IN 2020)
EDIT 6/29/2020: garc location for X/Y found (??), Soul Dew and 510 EV limit removed in ORAS (X/Y/gen 6?). You can also apparently enter more Pokemon than normally allowed (such as 4 Pokemon in a Singles 3v3). [510 EV Limit bypass not reached... sorry my mistake)