There are some threads about military nursing, I am not sure if they pertain to the National Guard or not though, you might want to try posting in the nursing career advice section.
I would suggest going to the army corps of nursing web site and contacting someone directly through there, they list some of the opportunities for both active duty and national guard on that site.
Other than that, I can't be much help, my husband is active duty and I do work on the base in a military health clinic with active duty, but I know more about the active side (which still isn't really all that much)! I am entertaining the idea of coming into the army on active duty myself once I finish school, I'm not sure yet though.
Good luck!