ok since you shouldn't use starch or fabric softener on these fabrics that they will sign I would use butcher paper on the back side to hold the fabrics stiff. This non-starched fabric will hold the inks better and setting the inks will be more secure. Than all you need is any flat surface to lay the fabric on to sign/write on and you won't need to spend extra on cardboard.
If you are wanting to do these in a plain white or off white color, you can buy the pre-done fabric sheets from
http://www.cjenkinscompany.com/ and that way you only have to cut them to size. I have used their sheets in lots of quilts and have never had any problems with them.
Then you can use some of the pictures from your wedding to add to the quilt and be able to fashion the quilt anyway you like.
I think this is such a great idea you have. But like with anything you want to make things simple so the guess don't have to worry about messing it up. You should also have a few extra blocks on hand in case someone does mess theirs up. Yep, it happens even with the best of plans.
Good luck and be sure to shows us the end results!