I would go with batiks. and do something semi scrappy. pick a pattern that you think represents them and then get lots and lots of batik scraps that you can use in the quilt. I would make sure that you are not going to wild and crazy but more in the same shades color in the same places. I hope that you get what I am saying. When I think of hippie i think more of little girls in gauzy dresses with flowers in their hair and nothing on their feet playing in the leaves and the groom in jeans and leather vests. I am thinking that they are more down to earth than a "real" hippie wedding. they sound like more outdoorsy people than hippie people. you could also back the quilt with a really nice landscape fabric that is woodsy. but goes well with the top you made.