I made a signature quilt for a wedding where the signature block was the center square of Variable Star block (COC). I stabilized with freezer paper and drew the seam allowance, and penciled in "do not write beyond this line" in the seam allowance. I agree that you need to try out markers first. The ink needs to be permanent and not run after washing. Also, it was nice having markers in several different colors - some people like to draw! And, quite a few extra squares, because people do mess up. We also set out squares of paper the same size for people to "practice" on first, and found that had been a good idea, because a lot of them needed to think through what they wanted to write and some even changed their minds several times.
Good luck with your project. I'm sure it will be beautiful and a wonderful wedding memory.