Looks goos mike.I'm not saying drilling 6 1/2" holes through metal, plus a 5/16 drain hole with a cordless drill is fun, but a C clamp can convert your trailer into a handy work bench!
Here are my wood haul trailer mods, I think it will work well with the pivoting sides. It was a lot easier to do than what I originally planned, and should work better also. Will keep you posted when I get to try it out.
I pictured the pivoting bolts being a bit closer together and the angle iron a bit longer.
If the angle was longer it would give a bit more leverage to lift the log. I see the limitations because of the wheels/tires, but a small notch(triangle) out of the part of the angle that the log will rest on and then weld it back together and they will be longer and go over the tires a bit. I was also thinking you could attach a little thinner gauge metal to the angles with teeth cut into them to grip the wood a bit.
If I'm not making sense let me know and I will make a little sketch.