I suppose you could come up with a custom classfactory, but you won't know anything about the data at that point.
I think there is also an itemRendererFunction property that might help.
Flex SDK Developer
Adobe Systems Inc.
what we did here was simply creating a SkinnableItemRenderer class which extends SkinnableComponent and implements IItemRenderer.
We basically copy the code from ItemRenderer and pasted on our class.
Then you can create your .as itemRenderers that will extend your SkinnableItemRenderer class.
We set the itemRenderers skins on the CSS file by simply setting a skin-class for each item renderer.
I hope this helps.