My Idea is to make this system completely secure, no matter what resources are spent cracking it. At least in theory. ^^

Are you familiar with the cryptsetup program? When used (with or without LUKS) it doesn't copy anything and definitely doesn't write unencrypted data to the harddrive. Instead it creates a mapping, a new device in /dev/mapper/, which can be accessed to read and write to the file. This means that leaving traces of unencrypted data is not a problem.

Acually I did some more research. Check this out (read the comments):
http://it.slashdot.org/story/09/04/30/201222/Forensics-Tool-Finds-Headerless-Encrypted-Files
It's about the same thing I'm working on, and it seems that my tool will work. ^^

Let me give an example:
I have one (1) 100M file that looks random (could just as well be a Truecrypt volume). When I open it with my program I say that my data starts 20M into the file, and enter my password. Then `dumpe2fs` says that the filesystem size is 30M. This means that my data is written between Mbytes 20-50. I can make the filesystem larger and add files if I wish to, but everytime I unmount the volume I use resize2fs to shrink the fs as much as possible.
I also have a "secret" volume between Mbyte 60-80. But can I claim that this is just random data, that can be used to expand my first volume?