Reminds me of previous times when software was sent via 'floppy' disk. Each disk had a unique installation/registration code.
If the purchaser shared the software with others then of course the registration code wasn't worth much. That would be the
same thing here when the user receives the hashed registration code.
Sure, but that means each copy of the add-in would need the OP to generate and insert a unique key (probably a GUID is best)

1. Ten people need the add-in

2. OP makes 10 copies of add-in

3. OP makes GUID in each of 10 and distributes

4. User 1 gets their copy, and installs it and the GUID in the XLAM is added to the registry (SaveSetting)

5. User 1 opens Excel to use add-in, add-in checks it's embedded GUID against registry (GetSetting)

5. Add in says "I'm good" and runs


But, ...

User 1 says, "Hey User 11. You'll like this"

GoTo 4

Without some central registration authority, I think it won't work