To be honest, after doing a bit of research, it's a bit of a pain to check ammo levels using methods. Hell, you'd need a signature to call CAmmDef::MaxCarry as it has no entity.
Incidentally, why would you need a TF2Items callback to get accurate numbers? Doesn't the post_inventory_application event fire after a player is given a new inventory? You could just iterate over their item slots then.
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Not currently working on SourceMod plugin development.