I forgot, my answer is for NAV2009 and earlier. I made these customizations in my addon for construction for NAV2009.
In NAV2013, these functionalities are by standard. You have 'Qty. On Job Order' value on Item Card as Job Planning Lines demand. Now you have except this, 'Qty. On Service Order', as well.
Also, there are new function 'Reserve' on Job Planning Lines. If you set items fore reserve, you can make reservations from Job Planning Lines.
I hope, I help you. You can customize old NAV versions or you can make upgrade on NAV2013.