Very interesting scenario. I like it a lot better than the scenario from Armageddon, although the obscure nature of the threat would probably be lost on most of the movie-watching public. The only problem is that a mass of neutronium below the Chandrasekhar limit (about 1.4 solar masses) would decay into protons and beta particles after a relatively short time. A single free neutron does so after about 15 minutes. I don't know what the rate of decay of an amount of neutronium the mass of Earth would be, but it would require an understanding of general relativity, a subject I'm still pretty fuzzy on. An astrophysicist could render a more quantitative analysis.