This won't help you legally. You may honestly claim that you don't have the passphrase, but you cannot honestly claim that you cannot decrypt the contents. You just have to call and ask your friend while you're detained at customs.
Ask the inverse: they decide to detain you, indefinitely, until you produce the encryption key or admit that there's some base level of contraband on your computer that you're responsible for. What happens in that case?
Maybe as a Canadian I see this differently. At least as an American you will have certain rights that you can hope to depend on in your own country. But I forfeit all those rights when I cross the border. What's to stop these guys from detaining me as an international terrorist?
Might hidden volumes help you pass a cursory check? Sure. But if someone really wants your data it's not likely going to keep you out of a contempt of court charge.
Now all that you have to do is prove that you either don't know or have forgotten the key that decrypts your random bits into a plan they will accept. How hard do you think it will be to convince an aggressive prosecutor/judge that you are innocent when you have no evidence (can't prove a negative) and the government has reasonable suspicion?
A small key does not contain enough information to make the data decrypt to any arbitrary thing.
Although TrueCrypt hidden volumes may have some practical applications, we think they are unlikely to be useful in the border search context because they are most helpful when lying to someone about whether there is additional hidden data on a disk. Lying to border agents is not advisable, because it can be a serious crime. [PDF: https://www.eff.org/sites/default/files/filenode/EFF-border-...]
This advice is based on the EFF document. IANAL.
https://www.eff.org/document/defending-privacy-us-border-gui...