I know that Jack Sobon has done something like this with a sperber mill (similiar to the alaska only with rollers). I believe the idea there was to start with a curved log with as close to the arc (or between the two arcs) wanted as possible, then bend a guide board as you indicated.
I assume you should be able to do this with your 14x20 but I haven't tried it and have always wondered curved you can get before the saw starts binding on the inside arc.
