One of the quilting shows had a blurb on this subject. Think it was E. Burns. Anyway, they showed someone somewhere making a barn quilt. They used 2 full sheets of plywood to make the bloxk 8' square. It was painted on the ground, then hoisted up into place. They had mounted something on the barn in which to screw the whole thing together. They used a cherry picker to get the boards up.
Sorry I can't be of more hels, but E. Burns has a book out on barn quilts, so maybe there's something in them about these. You could also try to Bing it!