Hello Dave
I've only done several day-walks in Shetland using a guidebook bought in the Lerwick TIC. There's also a section in Lonely Planet's
Walking in Scotland book.
Am not aware of any Shetland Coastal Way though I did find a web reference to the Shetland Access Strategy (Sept 2005) which said:
A continuous Shetland Coastal Route is perhaps a long term strategic goal
Can you share what you know about it please?