Bob Barnett asked:

Does "-dau" have a meaning?

I don't know the answer, but I strongly suspect not. Bok has described other songs where words and ideas came to him in his sleep, and he just transcribed what he heard. (See, for instance, "In the Kind Land" for another example of this happening.)

It's not an unusual phenomenon. It's just that it's a rare person who can put it to such good use as Bok.