Hmmm . . . that's quite a challenge!
I'm wondering, though, if a border will really disguise the stitches. How many rows are actually messed up (and how wide is the scarf)? I'm wondering if you could pick out the stitches from the bottom up to a certain point and then add your border to the good stitches.
I know picking out crochet stitches from bottom to top is tedious, but I worry that you won't be able to "hide" the loose tension.
Yeah, a picture or two would probably help me answer this (maybe).