I finally got the oscilloscope, and it does work. You can score one on eBay
for $10 plus big shipping - the thing weighs 45 lbs or so. I got a NIC 2090-III. More on that later.
My "Buster" test wire got a new upgrade - alligator clips. They replace the blade connectors, and they do a great job holding (as it turns out) as many as three resistors in parallel.
This time around, I followed my path from Experiment 2 and placed some aloftier resistance inline with the ignition coil outputs from the SMT6. All of these proved useless: 3.3k, 2.8k, 2.5k, 2.3k. None of those values allowed the ignition to fire. The value from the previous experiment (2.2k) is evidently the maximum useful resistance.
"Don't believe everything you read" is defined here!
* The ignition signal I found is not the suggested sine wave at all, but a square waveform as I suspected.
* While the service manual suggests that the signal should be 0v off and 1.0-3.4v with the engine running, it's actually around 0.2 off and 0.2-10+v (I'm guessing 12v peak - I didn't mess with further calibration of the scope).
Key: OFF ON ON ON
Configuration Engine: OFF OFF ON - HI ON - LO
Stock 0.23v 0.23v >10.00v 0.16v
SMT6 Ignition Input 0.20v 0.30v >10.00v 0.27v
SMT6 Ignition Output 0.14v 7.70v 9.50v 0.14v
SMT6 Ign Output w/2.2k 0.10v 8.00v 9.39v 0.12v
SMT6 Ign Output after 2.2k 0.08v 4.10v * 0.10v
The * above is for this note: The ignition "fire" signal starts at about 4.3v, and grows to 5.0v, where it stays for the second half of the signal.
Considering that the stock signals leaving the ECU when wired stock or when wired to the SMT6 (using the un-resisted pullups) are very similar, I'm prone to believe that no modifications are necessary on this end.
The other thing to note about that table is that Ign1 and Ign2 were completely different when the engine wasn't running. Ign2 was cooperative (this time!), sitting at a low signal, while Ign1 (used for the measurements in the table) was nearly the same as the "fire" level signal. I think I need to find the threshold for the ignition signal... that's next.