[ ... ]
Welding would almost certainly warp it. I would consider drilling undersized under a set-screw hole and using a dog point set screw engaging the hole. An alternative would be to cross drill it, use a taper pin reamer and then drive in a taper pin. Both of those would be less likely to warp the tube than welding would.
Torque will be limited by how thick the walls are. Flexing under load could lead to the shaft buckling, after which things get quite (undesirably) exciting -- especially if it were not designed with multiple pillow blocks between the sanding disks. (For that matter, I'm not sure how it would be used with multiple disks. Drums, maybe, but they would wind up with gaps for the extra pillow blocks.
That would be a pretty nice approach.
Is it possible to put extra pillow blocks between the sanders? That would help prevent undesirable excitement.
Good luck, DoN.