I think I would do a lot like what I think richard is describing.
I would move the log to the pit -if you can't roll the log itself, you can slide it on rollers. This is the methods I use most of the time for moving big logs, or timbers that are already square and finished.
Hew off the flat sides as you would normally, then shape the inside of the curve with an adze. Perhaps you would be best to shape the outside as the log lays on the ground in this picture.
For something like this, I would probably hew mostly by eye, maybe make a gauge to keep a fairly consistent thickness.
