I can't answer about the bark, however, I will give you my opinion on the sand. The reason we use sand is so the bark doesn't stick together and prevent draining or prevent the water from going into beds/pots. Have you ever watered mulch and the water just rolls off? Same idea here. I don't think the potting soil would do the same thing as sand since the soil can stick together itself unless it has other things added such as perlite/vermiculite (sp?). Before I started using sand I bought a huge bag of perlite from the nursery/seed and feed place for like $20; It's called a 50lb bag even though it does not weight 50lbs.
