Looks like a dead end. More than ten years ago I Began searching for S/W to interface with my 03 Chopper. At first, I felt confident about finding more information.
My brother has a degree in electrical engineering from the university in Wichita, KS. He actually looked up one of the designers who first worked on the computer design for Big Dog. Turned out to be an academic who got the attention of Big Dog because of his work on projects for the local small aircraft manufacturers around Wichita. By the way, this explains the lack of durability and issues with moisture. Computer wiring inside a 4-seater Cessna is not exposed to water and other external hazards.
Long story short: I believe the earlier computer designs done for Big Dog had design problems that were addressed as their ugly heads appeared, but still haunt us today.
This website - Big Dog Biker - has been, and remains, the only place to find historic information for our scoots.