Haven't actually tried (or needed) to tweek any ss coolant tubes. But back when I worked in a lab for a living, I worked with thin stainless tubes for gas chromatograph columns by simply filing the tubes with separation media (or dry sand). The stuff essentially becomes a particulate bending mandrel. Then you tape the ends closed so you don't spill too much. After bending you remove the taped ends and pour the particles out with a little selective tapping. Damn, that was a loooong time ago.....
As far as using the proper bending dies, I wrap hollow 4130 swaybar tubes with many layers of tightly wound duct tape so it fits snugly in the closest die. Seems to work at the low bending pressures & angles needed.