Yeah the encryption uses one that is present in 7.0 and that normally means is patchable at least to that, sometimes it can even go lower but there's a precedent in Stardew Valley online update that had encryption from 3.02 and couldn't work is FW lower than 6.0. I know for sure Link's Awakening works on 8.0 but i'm currently on 8.1 so i can't tell you if it can be made to work on 7.0. Only thing i can say is that is possible that the game doesn't go lower than 8.0.
If an NSP can be installed with gold leaf which ignores the update file, does that mean that game can be pacthed with your tool to work without the default update?