There is a much easier solution to all of this
1) Have a linux partition, have some basic junk in there (some PDFs, some presentations, some papers you wrote in 5th grade, whatever)
2) Have a storage partition for all of your music, movies, and pron
3) Change your menu.lst in grub so that any other boot partitions (mac or windows) don't show up
4) Make sure that your other partitions are not automatically mounted (this will be by default, but in case you modified your startup script)
5) Don't show the border agent how to mount partitions, chances are they don't know how to use linux
Done - you cooperate, they see you have a basic linux partition with open office and whatnot installed, and they are none the wiser regarding the other unmounted partitions. No need to encrypt them. They won't bother imaging your drive because there's no reason to suspect you. You were cooperative and showed them everything they wanted to see.