Can you match the color of the fabric that is lacking? i.e., if it's muslin that didn't quite reach the sashing, can you put a piece of muslin behind it? If so, I would do that and then very carefully (and with extremely tiny stitches) adhere both pieces together so the quilt top won't fray and the repair appears to be seamless. I would then make sure to quilt over this area also so that it is all secure.
Lastly, if you make a successful repair and you don't tell anyone it's been repaired, they likely won't notice it!
Good Luck.