If your on-site contractors cannot devise a good solution, their abilities are questionable.

That their abilities are questionable is now an unassailable fact - that is why I need help :)

You can't conceal a junction box.

There's no reason you couldn't, for example, build a "box" (some kind of hollow structure) below the beam and run your wires into that, terminating them in an accessible box.

Many pendant lights are available that are suspended by thin wire and can have the electrical connection off center from the hangar. You can then have a box installed adjacent to the beam and run the electrical wire there.