Okay, I got it working -- I couldn't get the new button thingy to work, but I was able to steal a line of code from that and replace the line in question in my quicktags file.
So I replaced line 371 with this:
edButtons[i].tagStart = '<a href="' + URL + '" target="_blank">';
and then tweaked it to look like this:
edButtons[i].tagStart = '<a href="' + URL + '" rel="external">';
Which look EXACTLY (I think) the same as what I did above. But, for some reason, cutting and pasting from your code made it work (?).
As far as the button goes, when I made the changes as instructed on the site that you referenced, none of my quicktag buttons loaded. So I clearly screwed something up. A couple of questions... I am supposed to *add* those lines, right? And not *replace* them? Assuming that I add them, I then end up changing the lines downstream (once I add lines 60-66, the line numbers below shift). So are the later changes referring to the original line numbers, or to those after the changes above have been made?
It would be handy to have another button for this, as I don't want every link to spawn a new window. So right now I have to hand edit out the rel="external" from those that I want to stay within the current window. Not a big deal, but not overly slick.