The first method would of course be the most practical way, but I wanted to know if it's possible by mission editing means. The less required addOns the better.
The second method would probably work too, but as I said, the ammo slider only works for the default loadout of the unit. The M16 magazine I talked about was only just an example, but take for example a magazine type that none of the units have by default and need to be given to the unit with the addMagazine command. In this case the ammo slider wouldn't work anymore, because the addMagazine command gives him a full loaded magazine which isn't affected by it.
But seeing as there doesn't really seem to be another way, I should probably just scratch the whole thing, unless there's any more ideas.