Jon, I tried to answer your question on the other forum but forgot my password. Then I tried to send you a private email (no dice, requires password). Here was my responce:
Adding the spark plug test function would be easy. You will need to fabricate a pair of brackets out of sheet metal that the plug can lay on. One of these brackets should be grounded and the other should be wired to the hot side of the coil (same connection as the test ring).
I will offer this however, I don't feel that the plug test function really tells you anything about the plug. It may fire fine in free air but not so much under pressure. That is why real plug testers have a connection for shop air. The plug is tested under pressure to replicate what it sees inside the cylinder. A plug tested under pressure is a valid test.
Your tester sounds like it will be neat and you have solved the bench clutter problem by mounting it on a dedicated stand. I like the idea of driving that using the treadle! Good luck
Don't know if this will help but it shows how the spark plug tester V brackets were mounted to the coil box housing on a tester. The grounded side on the left uses the spacer bolt and the hot side simply mounts to the insulated front part of the coil box with a copper wire attached to lead going to the brass spark ring or slip ring. Bob
The three terminals on the base of the coil tester are the "magneto", "Horn" and "Ground" terminals.
You can use these terminals to test/adjust a magneto horn by connecting the Horn and Ground terminals to a magneto horn.
You use the Magneto and Ground terminals to check the output of the Model T car magneto.
If you need a wiring diagram for a HCCT send me an email.
Ron the Coilman