Ok so I had a splash of inspiration. I think I'll use a square pipe flange on the post, then get a bit of metal pipe cut to the width of a board and threaded, then add a three way pipe fitting onto to tightly hold the board in place (add washers on both flange and three way split to spread force over more). Should be the easiest simple way to get a rotating piece that works I think.

Also means the 2x4 would be the limiting factor and can recycle the three way to feed the winch line.

Bottom of sketch is what I'm thinking. Also it should be sufficient to secure the pipe flange with lag bolts right? Again 75lb max load here

Thoughts?