Chaplains must both be an officer
and be ordained (or equivalent thereof) in whatever faith they are a part of.
Neo-Pagan is more of a catch-all term for the various paganistic faiths that have arisen or been revived in recent years then an actual religion, far as I (or at least the military) can tell. It isn't the largest part of the services, but enough are in to have a few Chaplains in a recognized Pagan faith (ie Wicca), and the military does attempt to provide a place of worship for us, but again: Neo-Pagan isn't the largest, nor the most....united? organized? group of faiths around. This causes a bit of problem getting both services and Chaplains of the Neo-Pagan faiths.
Hopefully they can start to fill in that gap, though.
EDIT: Here's a paper on the Neo-Paganism and the Military. It concerns mostly Wiccan branch(s) of Neo-Pagan faiths, however it holds the same problems to gaining a Chaplin in the armed services as any other one.
Witches 'n' Wizards 'n' Warriors, Oh My!