Hi Camryn,
Hmm, this is an odd one. I believe that it should be possible to do this for FLASH data, but that it hasn't makes me nervous -- I suspect but am not totally sure that the reason it's not implemented is because we don't want to run the risk of *not* getting results in some cells.
Your best bet might be to do your own derived field that looks just like the nearest neighbor one in yt/fields/particle_fields.py , but instead of feeding in the grid's particles, feed in the particles from an externally defined all_data() object.