Is it somewhat realistic?
I can put entire wiki page about what guns, heavy guns and military equipment is used in 60's/70's or 80's
Does it fit with the spirit of PZ?
Ofc we have army camps and even army bases
Would it make the game too easy?
Depends. Zombies in bases are offten in very large numbers and weapons still need ammo so it is easy to balance it.
Would it require rewriting the entire game (like changing the camera view, for instance)
Hell no. Meaby some more models.
Would it add enough to the gameplay that it would be worth taking the time to add? Yep a style of Play and items to Trade.
Now with Luisville or Raven Creek u need a lot of ammo if u plan to go there after highest pick of spawns.
Would it be found in Muldraugh, Kentucky common enough to be added?
Yes every military base and camp have heavy weapons. (light machine guns, mines or simple granade lunchers at least)
For zombies: Does it fit at least somewhat into Romero/Brooks zombie lore (the basis for 'proper' zombies)?
Yep