Now this is very interesting. Both because SCSI2SD is a bit expensive, and because the newer V6 boards would need an adapter to hook up to an old Mac. But on the flipside while the current SCSI2SD seems pretty swell for connecting to other SCSI devices via adapters; a Raspberry Pi itself is a pretty general reusable platform.
As far as I’ve been able to figure out, old Macs have ridiculously slow SCSI buses by modern standards of any mass storage device, and I think they didn’t even support DMA until the late ‘90s. But to be fair hard drives were typically in the 10s of megabytes in the late 80s – early 90s, and a few hundred megs at the most.