Hey Sean didn't I see Johnny Knoxville ride a rocket bike off that dock in a movie??? Seriously though, I would put two more piers in along side that dock and just raise the center up and put a header across for it to set on. If it still is in the pond that is.
As for the question at hand, I would say 1/4" rebar would work, just space it closer than what you would space larger bar. A 5 inch circle would be about 15 inches in circumference, 8 bars would fit at roughly 2 inch spacing more or less, tie them to every horizontal bar or use the slinky idea and you will have a steel structure that will be quite tough when suspended in crete. Just make sure you get your PVC in deep enough, I am gonna say 24 inches minimum to hold up to the ice. I worry more about when the ice shifts, a warm up in the winter can thaw the edges of a pond and the wind during such a storm pushes/pulls the remaining ice pack producing severe lateral shear. Many of these Texas guys have no idea how much force ice develops, not their fault, just geography has never forced them to deal with it. I can assure you the first time you see what the ice can do, you will wonder if you have them deep enough, as I said, I would also considering angling them to the center of the dock to help counter all this. Just my country boy idea on the subject. I still have 6 inches of soild ice on my pond, walked across it yesterday and jumped up and down on it, thaw is coming tonight I hear.
