Lovely!
Friends had a canvas up at their wedding and each person was given a printed square - with co-ordinates - and invited to go and paint that square on the canvas ... I gulped at the thought of all of the wedding guests in their finery handling paint (especially as I was wearing a cream jacket!) but it was very popular and the final picture, which turned out to be a picture of the couple, is gorgeous.
I think your idea of the blocks to sign is a much 'safer' one - to allow creativity and yet not to risk so much mess!
Signature blocks are a great great idea as people can include a favourite quote or a wish for their future together ....
Helen