From FWIW department: What you describe is possible, but is not very practical. A frame pack is awkward at best , and would be a real hassle on a bike. If you could mount it on the rack, all the weight would be pretty high. Even an inexpensive set of panniers would be a much better setup. If the objective is to day hike when you get to your destination a 5,000 cu. in pack isn't necessary. A small daypack with sleeping bag and tent could be placed on top of the rack. The daypack and panniers should provide plent of capacity, lower the center of gravity, and be a lot easier to handle. Try loading the pack on the bike for a short trip around the neighborhood, and he'll probably come to the same conclusion.

Is there a compelling reason he wants to go this route? My advice : take all advice with a grain of salt