AutoExec will do it too, register the event gainbuff to equip the arrow, and register the event losebuff to unequip the arrow. that way it will ONLY equip ONLY if you have the buff, and if for whatever reason you lose the buff, timing out, or accidentally shooting, it will unequip it.
Code:
<?xml version="1.0" ?>
<autoexec>
<register event="gainbuff_Unlimited_Shot">input /equip ammo "Arrow or Bolt of your Choosing";</register>
<register event="losebuff_Unlimited_Shot">input /equip ammo "Standard Arrows or Bolts";</register>
</autoexec>