Navigators are known generally as "the guy in the back" i.e. in top gun, goose was the RIO..Radar Intercept Officer. The Air Force has WSO's...weapons systems officers. Though with the F22 on the way, there aren't going to be many seats for nav's in fighter aircraft. I think all the other planes in the inventory with the exception of the B2 have Nav's. Riding in the back seat of a cargo plane didn't really appeal to me so I stuck with engineering.
If you want to be an officer in the Reserves, you still have to get accepted to OTS. Guard officers go to a commissioning source called AMS which is basically OTS compressed from 12 to 6 weeks. OTS is 2 years of ROTC compressed into 12 weeks. Guard and Reserve slots are hard(er) to get because you could work at the same base with the same people for 30 years, so they really take a close look at who they want to work with for the next 30 years.