I tried playing around with this a few months ago. What happened was... yes, they open up when dry. I tried putting them in a PVC tube, then letting them dry, but they just never seemed to dry out enough that way (casting fail). What did sorta work was wrapping a wet paper towel around them for a day to get them to soften a bit such that I could force them into a tube.
In the end, I settled on just letting them be bone dry, squaring them off with the bandsaw to a size that I could force them into a casting tube (even dry they have a little give). A few of the petals brake off, but it didn't seem to be noticeable in the final result.