Does anyone know what (if any) script runs when alternate ammo is loaded? (or even what script is triggered ~if any, when the '2' key is pressed).

Actually I want a way to determine in script what ammo is currently loaded, (Ideally to run script code when a certain ammo is loaded and again when its unloaded).

Using NVSE, you can use 'GetPlayerCurrentAmmo' to find the currently loaded ammo.

Sounds perfect. I'll look into it soon; If I can get it to work then my mod is almost done. All that's left is to make the sleepers lootable.