I also did a quilted wall hanging for my daughter's wedding. It was a small wedding. She was going to carry Calla Lillies so I found a pattern with two of these flowers on it. I used that paper pieced pattern for the center then made blocks to go around it. I did piece it before the wedding and quilted it afterwards. Yes, we did have one person sign it upside down before her brother realized what was happening but she was not upset and actually found it very memorable. I think she loves that that happened now. He was a great uncle who was well into his 80's and was very fond of her.
Yes to putting freezer paper on the back to keep the fabric flat and to keep it from "slipping". And yes to marking somehow the area to be signed although I did not need to since my blocks were already in the quilt.
I would also definitely have a large scrap of the signage fabric (or two if there will be a lot of people signing) ironed onto freezer paper so people can try out the pens and get a feel for how they flow. That was a huge help for people who were a bit timid about doing the signing.
I quilted the borders with words appropriate to the newlyweds and they now have it hanging above their bed. It is a bit "wavy" but they don't seem to care. We love her and her husband for the great people they are.
Good luck with what you are doing. When you are done, you will treasure that quilt forever no matter who writes what and where it ends up on the block!!! lol