USB is not comparable to SCSI, so a USB adapter for the SCSI bus is a custom device with a custom protocol and custom software needed to access it. This probably would be much too expensive to develop.
I don't think he is talking about a generic SCSI<->USB adapter, more like a device to fit onto the expansion connector found on the Blizzard 12x0IV boards. Anyhow, the development costs here would probably also be too much to justify making it, if it was possible at all. If it could be done it would've been damn cool, as this connector must be ? times faster than any clockport.
heh, DMA usb controller...that would have been an Amiga exclusive :-)