I've seen various 'copy protection' schemes for commercial 'audio' sic
discs, but not the one you describe, so I'd be interested to know where
the info is from.

Unfortunately I've long since forgotten where I found this, although I
seem to recall reading a reference on a website somewhere. There was
some discussion of this on an internal IBM newsgroup in 2002 when I was
already posting it as something I'd been aware of for some time. The
consensus of opinion then was that "Audio writers look for the periodic
servo error signal, and if it's not there, refuse to write the disk."

If you used "eccentric" as a search term then you're more likely to come
up with references to me personally than CD spirals :-)

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk