JSONPath or other XPath like utility for JSON/Javascript; or Jquery JSON
Definitely would be a useful utility.
My opinion is that the best way to approach this would be to stay as similar as possible to css selectors, as you indicate. I'd recommend looking under the hood at jquery's implementation of selectors.
I would suggest something like
var banana = object.function(jsonObect, "holding #Banana");var leftHands = object.function(jsonObject, "appendage[type=hand][side=left]");instead of your usage examples.
I'm not sure how the upcoming native json support will affect this...
OK, I have created a prototype for this, available here: http://code.google.com/p/jfunk/
It has already proven useful for me, so I will probably slowly improve and refactor it into something nice. But if I get good feedback, I can move more quickly. I would also welcome help.