By overhang I assume you're referring to the span where the chair is. This is a perfect spot for a cleat along the wall. With the cabinet holding the desk at the left, another cabinet holding the other end at the right, and a cleat supporting the entire back end along the wall, you'll have more than enough support for this span.

That said, do absolutely everything in your power to avoid having the panel seam fall right where you sit. That's the worst possible spot for it. You should be able to get an 8' panel section up your stairwell, even if it takes a bit of exercise to do it.