Try unplugging the power cable to the hard drive and booting the Mac from a boot disk that contains the hacked Apple HD SC setup program. Then once the Mac is booted, plug in the power cable to the hard drive. This prevents the Mac from loading the driver from the hard drive during boot (which probably causes the sad Mac) and it should still show up on the SCSI bus this way, allowing you to format it again. This should at least get you back to a working state where you can try your FWB formatting again if you want. Note that the SCSI ribbon should remain plugged in during this entire procedure.

There is a chance that plugging in the hard drive with the Mac already on will cause the power supply voltage(s) to dip down and crash/reset/turn off the Mac. The ideal way to do this is to put the drive into an external enclosure and wait for the Mac to boot before turning on the power to the enclosure.

Another solution to the power problem (if applicable) is just another computer, even if it's a vile old PC. Pull out a power connector from the other computer, plug it into the Mac's hard drive, and turn on the other computer when you want the hard drive to turn on. Crude but very effective.