This problem is easily solved by using Cryptfs by @kapitanpetko (who also has a great blog on Android security). Basically it uncouples the lockscreen password from the FDE password. So when you boot up the phone from a full power down it requires the long FDE password, but you can still have a relatively short lockscreen password.
Anyone using Android FDE should use this.
This is a cool project and if I had an Android phone, I would use it.
I wouldn't recommend that people use this, though.
Since the app has the user set up a strong (hopefully ~40 character) passphrase for disk encryption, which only has to be entered when the device is booted, I'm afraid that people would just forget this passphrase. Even if the user wrote it down and kept it in a place they considered safe, running out of battery while out and about would render the phone unusable, since they would be unable to remember the passphrase after plugging the phone in to charge.
There are also other problems with Android, unfortunately:
The EFF wrote a white paper on the various legal and security implications of crossing the U.S. border with devices.
They have a section on secure deletion, which outlines the various pitfalls of this approach.
They point out that a factory re-set of an encrypted iOS device or Chromebook is more likely to be safe, since the encryption keys are stored on the "secure enclave" (in the case of iOS) or the TPM (Chromebooks). So, any data remaining on disk would be encrypted with this securely erased key, thus (probably) rendering this information unrecoverable, even if you unlock this wiped device at the border (probably).
The safe solution for sensitive work is to not cross the border with devices that contain, or have ever contained sensitive information.
I'm perfectly aware that this is a pain in the neck (it's also a pain in my neck, I also like to get work done when I'm traveling like everyone else).
This particular problem is going to require a legal, policy, and activist response. There is no cute hack to deal with physical coercion at the border. Until then, encrypting and uploading your data, then transiting customs with either no devices or an utterly clean (not wiped) device that has never touched sensitive info, is the best way to deal with this.