I'm into making it as easy as possible but still retain the same elegant feeling of the quilt that you have right now.
I've used the fusible threads and the fusible tapes - I love them both! For a long distance, I strongly recommend using a ruler so you know the line will be straight.
The quilt is in shades of blue and white, correct? How about a 3-row line of threads (should be wide enough to cover the shadow) bluewhiteblue? if you think they won't be quite wide enough, use the somewhat wider ribbon instead of thread.
If you poke around the Kreinik site a little, they used to offer a sample pack - small quantity of a couple different products so you could play with a 'copy' of the problem spot on the quilt - take pieces of the same fabrics, sew together, add batting and backing, and sew the whole thing together around the outside edge. Now you can experiment to see what works best.........I love to experiment with stuff, but not at the expense of my finished project.
Please feel free to PM me if you've got any questions. I love problem solving. I'm sure if we talk about enough options, something in that list will be perfect to solve the problem.
That quilt is so gorgeous you have got to enter it! If the right solution can be found, you won't have to worry about whether the judges will see the shadow or not.
Vicky :D