i would use a bunch of commands to copy some pixels around from surfaces to your background.
because it is only horizontal tube, you can copy the texture for the background manually line by line ...streched to the right amoount of space.
edit:
ok, i see the video above post solves it nicely, a shader is faster than copy of surfaces line by line... so i prefer shader! (but the principle behind it is same as i wanted to do)