That makes sense. I’m not sure why I didn’t think of the contacts being the issue, thanks! I’ll try this again when I get home today.
My theory was that there was too much voltage from the transformer (as it was reading 28V) and that somehow the tiny amount of current flowing through my multimeter was drooping the voltage just enough to put it in the right range for the OSPI. I had tried the 5v/ground pins as well as the diode, and both measured 0V.