The version played on stream when they got to the E4!
Note: This isn't the definitive version, but it is the guaranteed version. There might be a later release with more bug fixes and more things added later. Also note that this is the TPP build, with things like the gender selection tug-of-war and forced nicknaming intact.
Apply to an unmodified English Emerald ROM. The following sha1 sums should match. You will need to use a BSP patcher, such as this one: https://aaaaaa123456789.github.io/bsp/
f3ae088181bf583e55daf962a92bb46f4f1d07b7 pokeemerald.gba
7ed9ca13716ddddc22a398fda532ef670095dc49 trihardemerald.gba (Normal Version)
8946fc25d184052c601d46fe7a9a91fa11e775a2 trihardemerald.gba (TPP version)
Update 2019-08-28: Built and added the Non-TPP version of the ROM, which removes things like the gender selection tug of war, forced nicknaming, and some level adjustments. The original BSP release is renamed to include _tpp.
Update 2019-12-03: Some cheat codes have been developed for the game, and can be found here.