Okay I am a bit weird on this - but you have a year to send a wedding gift (according to Emily Post) and you you don't have to tell them something is coming. They may not even notice you didn't even send a card. I would finish the quilt and give it to them with the card when it is done. Don't stress over it. The fact you are making them a quilt when you could have just bought something off a registry speak volumes.
Enjoy the wedding, no one is going to know if you sent something, gave them a gift card or a check or anything else. When you deliver the quilt, then you can explain why it's late, only if you want to because I am sure they will not care.