So, I ended up testing out about a dozen cast-offs.
If you're interested in the results, you may want to click on the image to open it bigger in a new window so you can refer to the picture as I talk about it.
Also, many of the cast-offs I tried out were found in an old post at Tangled Up in Yarn (thanks to Kristi for gathering all this information in one spot).
#1 Basic bind off (done in the 2x1 rib pattern). I'm sure you can all guess, it's nowhere near stretchy enough. I included it as the control subject.
#2 The 5th one in the post, called Peggy's Stretchy Bind-off for Toe-Up Socks. The trickiest thing about this one is that you have to set it up on the row BEFORE you bind off, by casting on extra stitches each time there's a change between knitting and purling. It's definitely stretchier than the basic bind-off, but it does NOT look good with the variegated yarn.
#3 The 4th one in the post. At least as stretchy as #2, but decidedly better looking.
#4 The last one in the post. I do kind of like this one. It's good and stretchy, and I like that it gives the edge a little bit of a wave, but the wave is centered over the 2nd knit stitch, rather than between the two knit stitches, so it looks a little off to me in the 2x1 rib. It MIGHT look better in another rib pattern. Some of the twist is loose in the yarn in places, which makes it look a little messy. That might be caused by this cast-off technique, because I'm not seeing it on any of the others.
#5 The 3rd one in the post. This one adds a little extra stretch to the cast-off edge, but not as much as I'd like. To fudge a chain one with just your knitting needles, pass the single stitch on the right needle back onto the left needle, and knit it through the back loop... but wrap the yarn the opposite direction from what you normally do when you knit.
#6 The second one in the post. I don't think this one is particularly stretchy, but I have to admit, I didn't quite get the pattern right. I forgot to pass the yarn back and forward.
#7 I chained 3 in between binding off the 2 knit stitches. This does offer a lot of stretch. It also leaves a gap... which could be considered kindof lacey, but I wasn't feeling it as a topper for this rib pattern, so I wont be using it.
#8 Another attempt at a picot crochet cast-off. I did it more like a true crochet picot, in that I slip stitched it to the base of the chain before moving on to the next stitch. It's less stretchy than the one before, but does look more like a picot. The gap is still there, though.
#9 This one is the knitted picot bind off (at least, it's my best attempt at deciphering the instructions for it in Montse Stanley's Knitter's Handbook). This one does have good stretch, but I think it's my least favorite of my picot attempts. The picots look too pointy, and they want to bend forward.
#10 The sewn cast off. It's the first one mentioned in the post, and it's supposed to be from Elizabeth Zimmermann's Knitting Without Tears book. I really like this one. It looks a lot (and stretches almost exactly as much as) the long-tail cast-on. I'll have to try a larger swatch on this one, though. I think the purples matched up on the stitches on the needles and the tail I was using to do the sewing. If that's the case, it may look kindof yucky when the colors don't match up. I hope I'm wrong, because otherwise, it's my favorite of the lot.
#11 Or should I say # 12. I had intended on doing a tubular style cast-off as my last experiment, but as hard as I tried (and believe me, I did... I was seriously trying to analyze how it worked, so I could modify it), I could not convert the instructions for tubular cast-off for 2x2 ribbing to do it for 2x1 ribbing. So, in the end, I tried another way (I was winging it, and I don't know if I could explain how I was doing it) to do crochet cast-off with picots. This was the best LOOKING of all my picot attempts, but it was not stretchy. I ran out of yarn too soon, so I can't compare it properly, but I don't think it's any stretchier than the basic cast-off.
The verdict? I'll be doing # 10 if the color thing isn't an issue, or # 3 if it is an issue.
And it looks to me like if I want a nice picot edge (like this one, or this one), that looks like scallops, rather than like strange bumpy growths, I'm going to have to work top-down, rather than bottom-up.