Craftybear posted a tutorial for a cathedral window pillow that had something in there that might help you with this. It's about 1/2 way down and what they did was to sew a couple of stitches to hold it in place while they did the curvy part. I think if you did that on the seamline, it might hold your piece in place and allow it to line up correctly. I'm attaching links. Will see if I can attach the picture too.
Here's the pic with the stitches to hold in place:
https://lh3.googleusercontent.com/-O...4/s1600/26.JPG
Here's the whole tutorial:
http://www.modabakeshop.com/2011/04/...w-pillows.html