Another thought would be to take your invitation to a printing place, think Office Depot or Staples, and get it enlarged, then get them to print it on your background fabric. I am thinking you would have to use freezer paper to attach the fabric so that it would go through their printer. Then, you could use fabric paints, or whatever to enhance the invite. fyi, I would take a couple of pieces or even 3 just to make sure you get a good copy on the fabric. This would work especially well, if they like black & white quilts with another accent color. You could take this scheme one step further and make it a signature quilt. It would be a forever memory of their wedding day.
Edited to add: some examples
https://www.google.com/search?q=sign...iw=982&bih=653