Well I went ahead and installed a cutout switch on the battery. After some digging around and tracing things back, I discovered the issue- The dome light. It is a push on/push off type unit, and I had used it for the first time the other night when I was blowing the drive as arrived after dark. Apparently, pushing off was turning off the light, but wasn't shutting off the current flow. Some sort of weird leak. sorted now.
Current draw when key is off is now zero. The ECU is also off, by the looks of it, until you turn the key on.
Mystery solved.