Re:TrueCrypt (Score 2, Informative)

. . . I think TrueCrypt needs to have an offset for its containers . . .

It does. The hidden partitions in TrueCrypt start from the end of the file (this allows you to keep the non-hidden partition intact at the front of the file).

When you enter a password, TrueCrypt first tries to decrypt the random data at the front of the file -- and, if that doesn't look like a TrueCrypt partition, then it tries to decrypt the end of the file where the hidden partition would be.

So... what you're proposing should already be doable. Create a standard file container of size "Video + N" that contains a hidden container of size "N". Once the hidden partition is set up, simply overwrite the front of your file with the video. TrueCrypt shouldn't complain, because random noise (from the encrypted, standard partition) and a random video file should be equally unintelligible from its perspective.