Planet13 - 2:46 am on Mar 30, 2012 (gmt 0)
Well, headspace2 seems to be working, but it is a bit of overkill.
It's one of those "all-in-one" SEO plugins. So there are dozens of different settings. Although apparently you can disable the ones that you don't want and it won't add to the overhead.
The other drawbacks are that you can't specify that the custom field input should ONLY appear on the Category admin pages (it shows up on the Post admin pages, too).
The other thing is that the input field is only kind of a small text field (it would be nice if it had it's own editor, but I guess you could edit what you want in the description input text box, then cut and paste into the custom field text input.)