NAPS04,
I graduated from the Air Force Academy preparatory school and completed a semester at USAFA before leaving due to academic reasons. I won't go into the details about the latter, but I can affirm that the VA can use your prep school service as GI bill credit if pursued enough. However, it was not common to receive the VA benefits... according to my other classmates who dropped out and are now attending civilian colleges. However, it really does depend on different situations. For example, many states have different attitudes towards it. I have a classmate of mine in Texas who is using a partial GI bill scholarship on top of his school's veteran financial programs to reduce his housing and tuition costs.
It took me a year to finally receive my VA benefits in my state but prior to that I was using my school's "veteran's non-resident fee remission program" to cut my tuition in half. (I go to an out-of-state college). Regardless, if you don't get the VA program there are still programs out there you can apply for. Doesn't hurt to ask!
P.S. I am unsure if NAPS or MAPS had different policies on this, but if they are the same as USAFAPS then I would say definitely try to pursue it. It's a complicated process, but if you have any more specific questions just message me and hopefully I'll be able to help. Good luck.