If we put write-in votes on the ballot and have them registered, they aren't really write-in candidates anymore. I think registering as a write-in candidate defeats the purpose of being a write-in candidate.
They would still be write-ins, their names would just not be on the ballot.
In our current system, it's too easy for a person 1 day in advance of an election to declare himself a write-in candidate, then win easily, without doing any campaigning or anything. The point of the fun in running for fantasy office is campaigning.