Much as I love the mattress stitch, it only works for vertical seams. That's why it's good to have the "Invisible Horizontal Stitch" in your bag of tricks as well.
With right sides facing, you simply insert your needle under the first "V" below the bound off edge on one side. Next,cross over and insert it under the first "V" below the bound off edge on the other side and pull snuggly. As long as you are joining to pieces with the same # of bound off stitches, you'll just work V to V all the way across. Studio Knits has a great visual tutorial on this stitch here.
Notice how my seaming yarn just disappears even on the underside? It's magic.
So, the Lizard Ridge Afghan is FINALLY all seamed. It wasn't really as bad as I thought it would be. Now to decide on the color for the border. I'm going to try an attached I-cord border -- sure hope I like it!