In the K-machine schematics, in the NuBus Spy Data Paths, there is a switch near the upper right labeled AUTOBOOT. If it is on, the machine boots from the boot prom. Otherwise, the machine powers up in a halted state and some other device on the bus is expected to boot it externally.
Writing is hard. My original post went into detail about we used a serial cable from an LMI Lambda to a NuBus debug card in the test rack. You could write the K-machine memory from the NuBus to load a bootable program, then you could single step the machine through the debug path.
(Ref: The code for the K-machine remote debugger.)
The nitty gritty details are there in the code.