Ok, so the combination of Truecrypt and ext2/3 is not optimal for sharing data between Linux and OS X. Truecrypt and fuse-ext2 on OS X cannot mount an ext2/3 partition read-write. So I will now go back to the lowest common denominator for all OSes: NTFS…
Update: Yes, it seems NTFS is a good alternative. With both OSes using NTFS-3G, I can use the Truecrypt file on both Linux and OS X — and even Windows, if I had one… Creation of a 200GB Truecrypt file takes time, though. And copying all the data to it, even more.