You can't (unless you want to store them next to the originals, a proposition I find untenable). Some of the scratch disk settings are stored at the project level (things like video and audio previews), but the one's you're talking about are global - one setting for the whole program.
I don't like that arrangement myself. ALL scratch disks should be stored per project.
Thank you so much Jim. I thought as much, but I wasn't sure.