Well, this thread has turned out like the Discover card commercials where "we treat you like you would treat you".
Cause of problem identified.
Main injector coil apparently becomes open for just a millisecond or so sometime after the engine starts, causing the ECM to think that injection has stopped and therefore cut power to the interceptor valve. I suspected something like this but I wanted to confirm before arbitrarily swapping parts from the one other Komatsu truck we have here that can serve as a donor, and especially before ordering any new parts.
So I made an extension lead of AWG 18 wire that I inserted into relay terminal #30 (feed out to interceptor valve) of relay # 6 in the EPA relay box on the truck. This extension lead allowed me to "tap into the circuit" and observe the voltage on the circuit, seeing the voltage disappear at the same time the MIL lit up. That was the moment the ECM received the signal that injection had stopped (setting code P1241) and in turn the ECM cuts power to the interceptor valve.
Now I just had to figure out what was causing the ECM to think injection had halted.
To that end I swapped the injector driver module from our other Komatsu -14 over to the ailing truck and the problem persisted.
Next, I decided to swap the main injector. Bingo! With the injector from the other truck, the engine would run just fine and never turned on the MIL.
But just to make the test a double blind thing, I put the injector from the ailing truck in the donor truck and as soon as I started the engine and touched the accelerator, the MIL lit up.
Connecting the reader to the truck showed it also had registered code P1241.
I erased the code and put the donor truck back together with its original parts and put it back to work.
Have an injector coming.