The weight may or may not be an issue, you won't know until you roll it across a scale. You're going to be doing extensive modifications to the interior anyway, so any cabinetry, etc you may remove to make room for your piano will partially offset the weight.
If you pull this off I can't wait to see the pictures. Short of cutting a hole in the side, how are you going to get this thing on board? I can't picture a full size upright fitting through the side door of a C. If you do get it on board, like others have said think about the inertia in the event of a crash or sudden stop. You'll want to secure it with hardware that can withstand at least 3 times the weight of the piano. To achieve this you may have to secure it to the chassis frame. The walls won't handle much.
