Threat model: you're attempting to enter a country through an international airport. You have reason for traveling which you cannot cancel, so you must enter the country. The local border patrol has demanded that you unlock your phone and laptop for inspection. If you refuse, you'll be refused entry into the country. What do you do?

Regardless of having something to hide or not, I do not want to give anyone access to an unencrypted file system, especially considering the fact they they'll take it to a backroom alone. Is there a feasible way to boot to an alternate operating system, even in front of them, such that they cannot tell you're doing anything unusual and they cannot tell that they're only seeing part of the hard drive?

There is always the obvious option of not traveling with anything you couldn't stand to lose.

@jdgregson VeraCrypt supports a hidden operating system which is perfect for that. It is impossible to tell that the operating system you provided the password for is the dummy "outer" volume or the true inner volume. However, general computer forensics could be used to make a case that the operating system you have exposed has not seen any recent use, but that would be circumstantial and would require a much more in-depth analysis.

However I'd really say you should not be bringing a device that you are forced to decrypt anyway.