Well - I posted an earlier reply, but apparently it went into vapor.
You are right about not being available. A google search on the part number just showed a few strange foreign markets. It seems strange that Honda would not make that testing harness available for the DIY owner. And, as you indicated, we are talking about different functions. The testing harness allows you to test whether a sensor is operating within specs, and allows you to determine whether a malfunction code is the sensor, the circuit, or the ECM.
As for being pricy - that also sounds strange. We're simply talking about three interface plugs for the ECM that allows you to directly access the electrical inputs and outputs on individual circuits with a volt/ohm meter. No proprietary stuff, there.
Which reminds me of another matter that has ticked me off since I purchased the motor. Honda has never seen fit to output realtime engine performance data to an NMEA 2000 standard, like just about all the other manufacturers. That is not a difficult task. I guess they are making too much money on those funky digital gauges they are peddling. Here I sit with a fully operational NMEA 2000 network on my boat, and cannot get any engine performance data on that network. That's disappointing!
Okay - I'm off the soap box for the moment.