First an unrelaited fact I got from a guy at Amitrix about a ram card they were working on.
The CDTV's diagnostic connector doesnt have autoconfig and they added the memory by having a timed delay so after x milliseconds any read from the autoconfig address is taken as being aimed at the card.
Now I would think there was a time out on the autoconfig process but as the cfg line is simply passed from board to board there may not be.
The second way is to assume before presenting the first board with a cfgin sig it looks at the autoconfig address and sees if anythings there. It would account for any CPU slot board being first on the list .
As for the ram CS line on the fast slot, still use it as that 128Meg is fixed and if not used you have lost 128Meg of address space for no reason.