For anyone who stumbles upon this, the offset is 0x27CC. Use a hex editor and change the value from 80 -> 00
This will revert the bottom screen to a black screen as it was pre-obtaining Rotom. Finally, it will no longer spam chat messages at you.
The FieldMenu save chunk located at 0x4600 seems to only be for the button menu that opens up when pressing X. There are a few stored values here related to Rotom's affection and Roto-loto (source: [USUM] Rotom Pokédex - Pastebin.com) but the actual flag to disable Rotom on the bottom screen is at 0x27CC.
