I could make a small program to send known data to the port and learn what it does by capturing the port output. Looks like there are 8 data lines, 1 IRQ, reset, ACK, A1 (address?), R/W, and data request. (15 Digital lines).
I just purchased all the used books I could find on Atari ST programming to see if they might have something (I also just wanted them).
Hopefully one will have some info on the hard drive interface.
I'll keep looking to see what I can learn about it. Let me know if you find anythig new.