What purpose do you need that for?

It would allow chambered rounds to be scripted in.

player setAmmo [currentWeapon player, (player ammo currentWeapon player)-1];
// do reload here
player setAmmo [currentWeapon player, (player ammo currentWeapon player)+1];

If the ammo count of the new magazine is 30/30 then you will simply be wasting a round.

Aaaah, clever :)

The only problem I see is that catching when a unit reloads is a bit hard, see this for more info: http://forums.bistudio.com/showthread.php?173330

I'll make another issue for a Reload EventHandler.

EDIT:
http://feedback.arma3.com/view.php?id=18265

From what I discovered, it is possible to catch a reload action (in a simple sense of the word) via

player addEventHandler ["Take", {if ((_this select 0) == (_this select 1) and {isClass (configFile >> "CfgMagazines" >> (_this select 2))}) then {
hint "Reloaded";
}];

Hell, that's quite a clever one :)

Could a 3rd param be added? Make it false by default, but when set to true it allows the magazine to be overfilled.

player setAmmo [currentWeapon player, 2000]; would just fill the magazine as it currently does
player setAmmo [currentWeapon player, 2000, true]; would set the magazine to 2000, regardless of the max