My daughter did this at her wedding. We cut 6" blocks of lighter blues and creams to match her wedding colors, and I'm putting darker colors in as I piece them. We didn't put anything behind or under the blocks to write on them and no one seemed to have a problem, but the sandpaper surface would have made it that much easier - good idea. I am now in the process of adding the stabilizer.
One very important thing to remember - the seam allowance!! Many people didn't think/know/read the instructions or pay attention to the box we drew inside the block (with tracer paper and wheel), indicating where they should write INSIDE the box. So now, I will be applique-ing the blocks onto slightly larger blocks with invisible thread, instead of sewing them into blocks. No big deal, but if you can find a way to help people stay inside that seam allowance it will be much easier later. Maybe an attendant at the table at all times?