Last I heard the NPS was trying to figure how to adapt the structure to contemporary occupancy. Modern accessibility laws (ADA etc.) and safety codes have to be addressed, and adapting a tiny historic residence like this can be pretty challenging.
Looks can be deceiving, and it isn't really vacant. US Park Police are all over the little building. Shouldn't say any more, though.
As for its historic significance, the listing you found is probably outdated. Researchers have learned a lot more in the last decade about the various lookout stations at Lands End, and I'm hoping the Octagon House will eventually be renominated to the National Register for its own significance and not as part of a Sutro District.