I agree with the advice on avoiding having anyone sign on a quilted quilt. You could have signatures done on separate fabric pieces then appliqué to the quilt. If hand appliquing, I had people sign on fabric pressed to freezer paper for stability using a Pigma Micron pen. I put lines on the freezer paper to make it easier to sign. See photo of one such signature after the quilt was completed. One guest used their own pen and it ran horribly, but I was able to recreate it by tracing it. I have appliquéd circles to a completed quilt (I thought it was boring and needed something), but nothing large enough for a signature. See attached.