I have no ideas to offer you Ruth, but this sounds wonderful. I hope you figure out and I hope to see what it looks like when you do. :)
On second thought, I might suggest that instead of trying pin leaves to the fabric, which might get weird once they start to dry out some and maybe curl on the sides, you might just cut out some different leaf shapes from freezer paper, press that to your fabric, and set it in the sun. Perhaps the freezer paper designs will cover the fabric well enough that the sun can't fade the fabric underneath? Just a thought. Anyhoo, good luck with this and keep up updated. :)