The bad news:
It's true, cats will prace accross hot coals to get to something
they realy want.
The good news:
Cats have a VERY SHORT attension span (we're talking major ADD here).
If you surround your harp with a "mine field" of scotch tape with the
sticky side up it will (most likely) stop the cats.
When they step on it, the tape sticks to there paws (they hate this)
and there attension is refocused from the harp to the tape.
You could also build an electric fence around your harp (check out Sears
farm catalog). It won't do perminant dammage; but, it's a good deturant.
Good Luck
Ron