Adding a SCSI Manager 4.3 "XPT layer" (for ancient machines) is not impossible or hard. FWB did it (loaded a clone of a fake SCSI Manager 4.3 on non 4.3 compatible macs). So many macs allow SCSI Manager 4.3, except for a couple powerc laptops, with system 7.5, that such a "dream nubus card" would not be too restrictive if limited to machines that run 7.5 and had a standard NCR 53C96 SCSI controller chip.

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Would you provide additional information on how one modifies SCSI Manager and puts it into firmware in a way where it can take over the SCSI Manager function? I know FWB did it on their JackHammer, but I have no idea how the did it.

The JackHammer works on 53C80 based machines too. I don't think the 53C96 is really a requirement. If one limits an expansion cards to machines with a 53C96, one eliminates all but the Quadras (and later).