quilt present and just tell them that your gift is not ready yet but should be soon (maybe give them an estimate of how much longer).
please, please do not stop working on the quilt, regardless if it's not finished by the wedding date.
i am not sure where you live, but where i am from, you have up to about a year of the wedding date to send off a gift. really, that quilt will be well worth waiting for, especially that lovely design you have chosen to stitch.
i would get the couple a beautiful card for their wedding day and pen in a line that "like your wedding day is 