Yeah, I also ask myself, if it is not possible to resolve the problem by adding some features to the register tool.
That would be the possibility to delete a key from an installed system and giving feedback online to the licence database (must be one or?). Of course, the key is deleted beyond recoverability.
Now, in the database exits the information that user x has safely deleted the key from his hardware with MAC adress y. The user can now sell his hardware and the purchaser can use (if not installing OS4) MorphOS only in demo mode.
User x can now install MorphOS demo version to his new hardware. Running the registering tool, the new MAC adress can be connected to his name in the licence database and a new licence key is generated. In this scheme, everything would run more or less automatically and releaving extra work for the developers, except for updating the registering tool.
Only problem could be backups of the key of user x. But this would be useless, when updating MorphOS and the installer also would have the ability to check online for a valid registered key. So there would be only "sniffing" when updating.

Well, just an idea...