Here's a sidecar frame that's available from DMC Sidecars for $1595.00. That's basically the same type of set up I used when I built mine way back when, only I used an air shock for the suspension. DMC has several different bodies to go on the frame and other makes of sidecar to choose from as well.
Looking at the round tube on the front of the frame above, it looks like you could come up with a torsion bar that would slip inside the tube and rig something up that way. A torsion bar was what I had in mind if I was ever going to build another one. Make a suspension like on a midget racer - adjustable static height, easily changed spring rates and nothing sticking up in the air next to the chair. Frame above is pretty simple, however. Wouldn't take much to work that out. Especially since I've already got mounts welded to the frame of the 900. Be a lot of fun sliding around on the icy roads we've got now. Maybe Prof. Flood will post a little clip of him sliding around the college parking lot - can't tell me he hasn't tried it yet.