I let this question mull in my mind for awhile. Then I remembered a trick I had seen Gayle Roehm use in her "Sssinuous" scarf pattern, Knitter's Magazine #117, Winter 2014. As you might guess from the name, Gayle's scarf twists around and back on itself. Because it is a scarf, a tidy slipped-stitch edge is appealing. After all, most of the selvedge is visible in the finished accessory. But there are places where the scarf needs to be seamed together. And the scarf will be seen on both sides, so reversibility is highly desirable. I am pleased to report that Gayle has thought this problem through and solved it!
In this video, Gayle show how to graft live stitches to the slipped-stitch selvedge.
And in this second video, Gayle shows how to pick up stitches. (A big thank-you to Gayle for posting these tutorials on YouTube where everyone can find them!)
Some of you may already have figured out how this will figure into a seam. Use Gayle's technique for picking up right down the middle of the slipped-stitch chain.
Then work mattress stitch using those picked-up loops. I've inserted bright paper behind the seam to make the loops more obvious. You can see I've basically just laced the two sides together.
While this does not produce the same inconspicuous seam as mattress stitch, it does produce a seam that is tidy. (I've turned the photographs 90° because they fit better on the page when horizontal.) The top image shows the seam from the knit side. The bottom image shows it from the purl side. The slipped stitches mean you have one stitch for every two rows, which is why the result is not the invisible seam mattress stitch can achieve. Still, not a bad solution. And I must admit, I rather like it from the purl side.