Very cool, OP. I work with a guy who helps run Appleseed shoots. I ought to ask him about attending one someday.

Some tricks for NPOA in Prone (this assumes the shooter has a rifle with a good bicep sling):  

1.   Get into the prone, align your sights on the target, close your eyes, adjust your body until you feel that sweet-spot, "that's it!" feeling of a good, bone-structure-supported-with-no-muscle-tension NPOA.  Then open your eyes.  If your sights are not perfectly aligned on the target, gently rotate your body around the elbow below the rifle instead of muscling the rifle onto the target.  Keep the elbow under the rifle in the same place like there is a steel stake driven through it into the ground.  Do this several times until you are ON that target every time you open your eyes from your comfy position.  

2.   Next:  If you have a proper NPOA in the prone with a tight sling on that upper arm, you can relax all your muscles and your sights will move only a slight bit.  

3.  Finally, If you have a proper NPOA in the prone with a tight sling on that upper arm, and you relax all your muscles and let your body go limp, your sights will move ONLY in the vertical plane, either slightly up above the target or slightly below.  If your sights slide off to one side or the other, you do NOT have a proper NPOA.

4.  If you are shooting at a row of targets and you are shooting RIGHT-handed, get your bone-structure NPOA on the RIGHT-most target.  That way, when you shift fire to the left-ward targets, you're kinda curling into your prone position and using more bone than muscle, instead of opening up your position to the right which requires more muscle than bone.  Muscle twitches and moves; bone is rock-steady.

If this doesn't make sense, try it with an empty rifle at home and use a target 10-12 yards away (a black 1" square on a post-it note stuck on the wall works great).

If anybody tries this in the next couple of days, please report back.

Couldn't have described NPOA any better

Congrats and nice job.

I was planning on attending a shoot but it's slipped my mind for a while now. My Garand is the only rifle I have with decent irons, though. If I use my AR I'd feel like I'm cheating by using the ACOG or being handicapped with the MBUS set I have in my parts bin.

How much ammo is expended over the two days?

If you are over 40 and your eyes are starting to go downhill, there is no shame in using a scope.  Better to use a scope and shoot up to your ability than use irons and get frustrated.  I used to have magnificent eyes and at 30yo could shoot empty 308 brass at 25 yards with iron sights without fail, but the human eye gets less flexible in the lens with age.  "A man's gotta know his limitations."

I cannot speak to that since it has been a while since I started with Appleseed (I was one of the original instructors in 2006) but we did tone it down as 7.62 ammo supply dried up and as .22LR dried up a few years later.   DRY FIRE DRY FIRE DRY FIRE will do more to make your live rounds better than merely blasting away with 300 live rounds in a day.