custom JSON sort_keys order in Python

Is there any way in Python 2.6 to supply a custom key or cmp function to JSON's sort_keys?

I've got a list of dicts coming from JSON like so:

[
  {
    "key": "numberpuzzles1",
    "url": "number-puzzle-i.html",
    "title": "Number Puzzle I",
    "category": "nestedloops",
    "points": "60",
    "n": "087"
  },
  {
     "key": "gettingindividualdigits",
     "url": "getting-individual-digits.html",
     "title": "Getting Individual Digits",
     "category": "nestedloops",
     "points": "80",
     "n": "088"
  }
]

...which I've stored into the list variable assigndb. I'd like to be able to load in the JSON, modify it, and serialized it back out with dumps (or whatever), keeping the orders of the keys intact.

So far, I've tried something like this:

ordering = {'key': 0, 'url': 1, 'title': 2, 'category': 3,
             'flags': 4, 'points': 5, 'n': 6}

def key_func(k):
    return ordering[k]

# renumber assignments sequentially
for (i, a) in enumerate(assigndb):
    a["n"] = "%03d" % (i+1)

s = json.dumps(assigndb, indent=2, sort_keys=True, key=key_func)

...but of course dumps doesn't support a custom key like list.sort() does. Something with a custom JSONEncoder maybe? I can't seem to get it going.