I really liked the soap I made using rose petal infused oil. I gathered the rose petals from my rose bush, dried them, and infused them in oil. I have not used the petals on or in the soap, however. But I have seen rose petals soaked in clear soap (Melt & Pour) that can then be used as 'Soap Petals' . Creating soap petals that way isn't difficult, but seems just too fussy for me, however, they would certainly make a pretty little display in a guest bath.
Lavender petals do turn brown and over time as the soap looses more and more water (if you let if sit unused long enough), the buds can become very scratchy. In fact, I have found that many botanical exfoliant additives will become more and more scratchy the older the soap gets. My skin may be more sensitive than others, but for my skin, I'll only use exfoliant soaps on the soles of my feet.