So just a quick update for any other people who are trying to get a mac plus booting with the scsi2sd who come across this thread in the future:
I kept getting errors when trying to format disks larger than 1gb with the vanilla Apple HD SC Setup Utility that came with my System 7.0 disks (it identifies as vB1-7.0), so I decided to try a few other options to see if I could get the ideal 4x2gb partitions working.
Leaving my first drive 500meg I tried creating 3 other 2gb drives with Lido 7.56. When I tried restarting I got the reboot loop with the clicking noise and the weird screen patterns. I then tried tried booting (by using a floppy boot and inserting the sd card) and then wiped the 3 drives (including deleting the driver partitions) and restarted... I still got the reboot loop - so Lido is some how altering the driver partition on the first drive - I'm not sure it's all very odd, but I could have a) clicked update and not remember or it's updating them when it scans (which would be a very odd thing to do).
I then tried again, wiping the sd card and setting up 4x2gb drives with the scsi2sd utility. I then booted and tried formatting the first drive with the patched Apple HD SC Setup Utility v7.3.5, after a few hours of waiting for the drive to verify (Lido skips this so it's much faster) and rebooting, I got the same clicking/reboot loop.
So it seems that Lido 7.56 and the Apple HD SC Setup Utility v7.3.5 both install the newer scsi drivers that won't play nice with my Plus.
I'd be really interested in other Plus/scsi2sd users trying to initialise with the un-patched earlier version of the utility and see how they fair. It could be the key to getting the Plus booting with the scsi2sd with all the rom variants.