In my field we run up against copyright issues all the time. Some make perfect sense; some take a bit of pondering to discerne. It's amazing what you can get a copy right on. Yes, you can copyright an idea. Some medical researchers have done just that to prevent another firm from taking the next leap and capitalizing on their work. Sound pretty bogus to me, but with the right lawyer you can get anything done.
Saddly, you can loose your ass if you step on the toes of an inhospitable holder, however innocent (or ignorant) your intentions may be.
As musicians, and performing ones at that, you cross that particular line every time you play at a club and get paid. It's too hard to enforce so nobody bothers, although I know of one case where a record rep happened to be in a bar, in a bad mood, and threatened to file suit against the band and the club owner.
I'm not defending Mr. PRS, or holders of other seemingly bizzare copyrights, but his dragons are nice works of art and as an artist I'd copyright the bejesus out of them too. And as musicians, if you come up a pretty cool lick and flesh out a song: record it and copyright it.
All that said, I'd still prefer a dragon designed and crafted by the fine folks at Alembic.
adios,
Mike