@drumnj Well I can only speak about my environment. In our case we do not setup the netboot as the first option. I do require a tech to sit in front of a computer before it gets imaged. During initial power up we utilize the boot menu, this requires the IT tech to press F12 to get to the boot menu, where net booting is selected. This prevents accidental imaging a machine if everything was set to auto.
Now I can speak for Dells, they have CCTK utility that you can change the bios boot order, or setup on the next boot boot to the network and then return to its normal boot order. But you are right you need to have a booted operating system to run the utility.
But if you think about workflows. You really have 2 that come to mind. The first is you already have a deployed fleet of computers. They are already up and running on the network. From there you can use the hardware manufacturer tools to change the boot order. If they are windows based you can use GPO, SCCM, or third part tools like PDQ Deploy to push out the utility to the target computers to reset the boot order.
The other workflow is new out of box, or in a restaging queue. In this case there must be an IT tech involved. So when they have hands on the device they can change the boot order (as well as probably update the firmware on the target computer) before its imaged.