"... the El Reg storage desk thinks this is bonkers..."
OK, so you're probably not going to be able to create a working hard disk by painting a platter with this stuff...
However, if a way can be found to embed an SMM in a magneto-optical substrate, where the alignment of the molecule can be changed by heating the substrate with a UV laser, applying an external magnetic field to change the alignment of the molecule as it "floats" in the melted substrate, then turn off the laser to let the substrate cool and lock the molecule into its new orientation, then you'd probably have a pretty good shot at making an ultra-high-density archival medium. (The short wavelength of UV light would probably be necessary to achieve the desired storage density; it is likely that not even blue lasers would be fine enough to manipulate domains this small.)
The key to all of this is finding -- as the article indicates -- an SMM that can maintain its magnetic moment at room temperature. Since the uranium-based molecule mentioned in the article needs to be kept at 2 Kelvin to make this process work, it seems highly unlikely that it would be capable of being used in a phase-change-based storage system, much less a GMR-based hard disk drive. On the other hand, there are a number of other materials, such as certain samarium compounds, that may work for this kind of storage. (Some samarium compounds can maintain their magnetic moments at temperatures approaching 980 Kelvin / 707 degrees Celsius.)
Of course, when it comes down to it, if you're trying to pack that many bits into that small of a space, why not find a way to shrink a nanometer-scale lithography machine down to the size of a hard drive? Who needs magnetic or optical storage, when you can insert a cartridge of raw silicon, hafnium, and germanium, and carve your own chips? :-)