WG, The idea of adapting the speed bag for a person in a wheelchair is really a good one. looks like all you need to do is hang it lower, so the bag is with reach of you. since you could roll up to it and position yourself facing the bag (looks like the same shoulder and chest position as someone standing...). You can get a modest speed bag setup pretty cheap.
and wow, that guy is frickin' amazing on the speed bag! I had no idea it could be used like that. I also found this speed bag video clip on putfile.com
http://www.putfile.com/media.php?n=Speed-bag_Hallelujah
too funny.