The human mind is a wonderful thing, it ticks away on problems even when you're not consciously thinking about them. I wonder if I may have answered my own question.
The Millennium has a little shaft encoder on each axis to generate pulses whenever you move the needle in X or Y. These are obviously fed to the stitch regulator so that, the faster you move, the faster it stitches to maintain the pre-set number of stitches per inch.
So, now I'm thinking I don't need to make any electrical connections with the Millennium beyond "start/stop", "needle up/down", that sort of thing. Time to delve into the machine handbook and see how much info APQS divulge...
But I'd still appreciate any ideas/thoughts you may have. We can't afford $13,000 for the "official" mods. That's almost the cost of the machine over again.