The SCSI2SD, ZuluSCSI, and other SCSI boards can be modified in order to support the X68000's HDD activity LED without any internal wiring. On official X68000 hard drives, pin 34 of the SCSI cable is pulled low to indicate hard disk activity.
SCSI2SD
Cut SCSI pin 42's connection to the SCSI2SD PCB, and then run a wire from it to the cathode pin of the external LED header.
It is OK to also cut pin 33 in order to access this pin, as it is a redundant ground connection.
ZuluSCSI
The ZuluSCSI can be modded identically to the SCSI2SD, except that the LED output on the ZuluSCSI is inverted.
Rather than directly connecting to the LED header, install a 2N7000 as shown. The gate should be connected to the LED anode, the source to the cathode (ground), and the drain to pin 34. Other N-channel MOSFETs will work as well.