Yeah, it is always fun to post / see that drawing, well at least it is for me- I really like the 'slow- blow' feature of the bolt.
And it is a valid testing method too; take out the fuse, replace it with one of those replacement items, remove the battery and instead use a DC, constant current welder (stick or TIG power supply, not MIG which is constant voltage, at least the short- arc MIG welders most people can get to), set the machine for 200 or more amps, and turn it on. Wait a bit and something will glow and melt- there is your culprit! If inside of something else, such as a partition wall of a house, it will take longer to find but just as easy: watch carefully exactly where the firefighters are paying the most attention to and sending the water and again, you got the problem!
But back to your situation, I have been chewing on an idea to limit the amount of current going through a circuit so that you can power the system up and keep it powered up while you disconnect different things. The crude but inexpensive and very easy way of doing that is to use a tungsten light bulb, something like an taillight or interior lamp from a car would do. Just put the two legs of the bulb where the fuse would be and the light will turn on and should stay on; then start pulling component blocks off the harness and when you get the right one, the lamp will go out. Simple, cheap and absolutely valid as a testing tool.
I really cannot think of anything 'slick' or clever or anything that will reduce the troubleshooting time and difficultly. You are just going to have to remove things until you find the source of the short, and then further chase it down to the exact cause, such as will be possible. For example, if a wire is shorted going from the ECU, it is going to have to be pinned out and tested, which will be tedious at best. If it appears to be the ECU itself, you really will simply not know for sure unless you swap it out for another and of course that is going to be expensive and a little risky 'cause if it is NOT the ECU and you swap it, you may fry the trial ECU also.
Sorry to bring this as my best idea but I cannot figure out a way around it. ??
Brian
If the "Burn Through" method was feasible I would give it some thought. But, no, that's not an option for me. Nice try.