Success!

Thanks for all the input on my matching dilemma. After thinking it through, I came up with an approach that was probably more complicated than it needed to be, but worked beautifully.

First I folded over the selvedge on the left piece and matched it to the repeat on the right, pinning along the entire length. Then I basted it down, stitching as close to the fold as possible, and pressed it to create a crisp crease at the fold.

Then I turned the two pieces to the wrong sides, and pinned in the seam allowance close to the stitching line.

Working in two-foot long sections to avoid shifting the fabrics too much, I went back to the front side and removed the basting. Then back to the wrong side, I sewed a seam just a hair to the right of the crease I made with the iron. It also helped to use the holes left by the basting thread as a guide -- all I had to do was sew right between them.

It's pretty hard to tell in this picture, but the seam is flush with the right-hand side of the piece of paper at the bottom. There are a couple places where I'm a few threads off, but for the most part, the join is imperceptible. Pretty cool!