I put the ninja belt (target cursor) in a hotbar slot next to my shuriken counter. I click on it and target the shuriken slot to reload.
The only problem with doing it this way is if the belt goes totally empty it stops working, so you have to do this every few shots to be sure. If the belt's empty it thinks you want to throw rather than reload, which is just silly coding. It should default to use = reload when empty.
The single-click way is to use the shuriken as a stored target of the ninja belt, but this stops working when that particular stack runs out.
As far as I know strictly context-menu options can't be Commanded.