I recently completed a signature quilt.
The pigma pens worked out perfectly.
Like you, I was skeptical about doing all of that work just so it would wash out.
To test, i wrote with the pen on a scrap piece of quilt fabric. I poured laundry detergent directly on it and scrubbed like grandma taught me!
I let it soak for 2 hours in warm sink.
I figured that was the worst it would see.
The writing was still on it!
I could not have been more pleased with how this signature quilt turned out.
I made mine look like a brick wall. The bricks were 4x7 cut, 3.5x6.5 which was enough room for a big signature or a message.
This quilt has about 180 signatures.
One other thing I did - I added blank bricks throughout in case I'd missed someone... then, it could be signed after I'd quilted it.
I hope this helps.