There is a tool that is used to decrypt DSi's NAND and edit it, and there was some talk about it working on the 3DS, but no confirmation that it really does. Has anyone tried it?
TWLTOOL thread: https://gbatemp.net/threads/release-twltool-dsi-downgrading-save-injection-etc-multitool.393488/
TWLTOOL thread: https://gbatemp.net/threads/release-twltool-dsi-downgrading-save-injection-etc-multitool.393488/