Hmmm interesting idea but...
If you had a real custom 1.3 ROM with the SCSI.device, that would mean that the A600 could boot into a WB1.3 system using the internal IDE.
I guess we need some clarification from Jens/Oliver if ACA630 works under 1.3 workbench, cause if it doesn't (which is more likely) then you can't use ACATune to kick into 3.1.
Even if that works fine... and you have a small partition that will just kick 3.1, you'll need to have another partition with the 3.1 system that would need to be chosen in 3.1's Early Startup Menu, or else it would try to boot to the 1.3 partition = fail
As I see it, a nice and easier solution would be an 1.3/3.1 custom kickstart, switchable via a +5V switch, and a switch for Enabling/Disabling the ACA. That could also be 1xMini Dip Switch which can be easily putted in the case (thought about it for future but we'll see).
That can be made with only 1 switch as well probably (0=Kick1.3+Disabled ACA, 1=Kick3.1+ACA) but I don't know how to do it electronically