What a wonderful idea.. I'm sure she did just about cry. I guess the pattern you choose will depend on how much fabric there is to use from the gown..
I think something simple, and something that leaves the gown fabric in as big of block as you can, would be best. You'll want to allow the dress fabric to stand out, if you cut it into pieces that are too small you'll lose it.
Perhaps just a simple checker-board style quilt with one cord and one gown alternating. Since you have the cord already cut into 6 inch blocks you could see how many 6 inch blocks you could get out of the dress... Alternate them with the fine wale cord and put the heavier wales on the back.. Nothing says that you can't have a pieced back.. That's what I'd start with anyway.
I'd love to know what you decide to do. It's very precious to her and I applaud you for making something so special for her.