runner beans actually produce a
tuber and would be perennial where the ground doesn't freeze. I believe
I read that these tubers could be saved and replanted in the spring

This is true. We're on our fourth year of automatic beans. I don't know if
it's botanically a tuber; it looks like a big woody potato peeking out of the
ground. We have never dug them up. That might be a little risky, but if you're
in a climate witth freezes in winter you have nothing to lose by trying.