You might try a torch with a small tip. It will likely take several applications, but the technique is as follows:
Heat one small spot (where the inside of the bend will be) to a red heat, and quench. Let it normalize to ambient temperature. The tube or shaft will first bend away from the heated spot, but will wind up being bent inward at the heated spot. Repeat, and move the spot as needed to smooth out the bend and keep it in the same direction.
The underlying principle on why this works is that when you heat a steel material it expands. But when you heat it to the point of glowing red it softens and thus yields in compression. Upon cooling that area has been compressed and is shorter in length which pulls the part in a bend. This technique is very useful for straightening shafts, but should work for tubing.