First, regarding an SPDT reed switch, it has both an NO (normally open) contact AND an NC (normally closed) contact, so you can wire it either way.
Just wire to the NC lead and leave the NO lead unconnected and you have the equivalent of an SPST-NC switch.
Second, for using an SPST-NO type, I assumed you would not completely remove the magnet from the system in the ON mode, but rather you would re-position it (slide, flip, rotate, or whatever). Under such a scenario, it would not matter whether you used NC or NO. I see why you desire the the NC option if you plan to remove the magnet entirely. I would worry about the magnet getting lost during the extended time it is not being used, however. Would it be possible to tether the magnet to the system? Then you could use an NO type switch and not have to worry about losing the magnet, as a bonus.
It is possible to use the NO version and reverse the logic by adding a few other parts, but I can’t think of a way to draw less than a few tens of micro-amps continuously in this scenario. If that is too much current, then using the NC switch seems like the only viable option.