It’s not a wheel bearing or axle, you would hear those with the brakes off. I’m pretty sure it is in the brakes, but your mechanic had to pull the calipers off to replace the wheel bearing so he should have seen anything that would cause this noise.
I think your mechanic should go to his AllData and printout an exploded diagram of your brakes, then pull you calipers and see if anything is missing, like maybe an anti-rattle clip, or a clip that is backwards now. I don’t think this will be a problem for your trip, just an annoying noise. The only damage will be to the part installed incorrectly and that will not be an expensive part.
On second thought, I think the noise maybe one of the SS slides in the calipers that the ends of the pads slide on. If one of them comes out of position, then it will slide with the pads when the brakes are applied instead of the pads sliding on them and that could make exactly the noise you have and it could damage the rotors if left like this too long (5 - 10k city miles). There maybe a small groove in the rotors by now, but a shallow groove won’t hurt anything.