Gil,
All the trigger guards I've had were dead soft steel. I imagine there are some that might be casehardened, but haven't seen any.
My approach would be to take a piece of wood and cut a shallow V in it that would touch the guard loop at the points you believe are the end of each side of the flat area. Clamp the wood in a vise with the V up and set the guard on it. Take another piece of wood of appropriate length and size to make a drift to fit in the loop for pushing inside the loop, forcing the loop to bend in the V form block, and sand a radius on the end. The radius just needs to be smaller than the radius you want to end up with. Don't over think it. Just sand a radius on it. Then I'd take the wood drift with the radius and hit it with a mallet when placed inside the loop. It won't take much to bend it. Go light first and check it.
I would not heat a guard to bend it.
One thing about cold forming even soft steel, there is what's called "springback". This is when the metal is bent to a certain shape and then pressure is released and it returns toward it's previous shape. If your bend does not take the metal past it's yeild point, it will return completely to the original shape. Even if you take it past the yield point, it will return a certain amount.
Like any metal fabrication process, there are many ways to the same end. This is just my way.
