Just head to the hardware store and grab an $8 to $20 section of beaded panel, like the stuff used as decorative molding under chair rails:
"Run wiring behind a beadboard panel in the knee-hole at the back of your desk. Velcro the panel to blocking mounted on the rear legs or sides of your desk so that you can pop it off to access the outlet."
The end result is a seamless look with mock "molding" finish, and a tangled wire mess that's completely hidden from view. And with a few Velcro strips, your mine of cords is still totally accessible.