Hamish Henderson's poems are often in a meter that's ballad-like, and which fits a song.

The trouble with setting most poetry to music is that a lot of poems use a much more complex rhyme scheme (and rhythmic structure) than song typically requires. That Hugh McDiarmid poem above is a great example: the second line ending "goun" doesn't actually get rhymed until the seventh. And in between the two there's so much variation of rhythm and rhyme that it would sound very forced and contrived when sung. Not saying it can't be done though...

There's a good website out there somewhere which compiles a load of "Literary Ballads". Not hard to Google.