Bit of a bump, but just to bring this full circle...
Haven't had time or money to make an interposer board to signal snoop, not sure if I'll bother. I did snag a couple of those official Apple printer cables and they came in just now, and the behaviour between the official cable and my attempts at rewiring a PC cable yielded the same results. Whoever said it was right - I guess IBM wrote (or commissioned Canon to write) a totally new ROM with no backwards compatibility for the 1080A print codes. Disassembling the 1080A ROM to hack it into the 3852 memory map is not my forte either... I guess the hunt is on for a proper 1080A.