after a sort/filter, I'd like to apply special styling to the item in the new 'first' position. Is there an existing method for getting items by their post-isotope position index (which is now different than their DOM ordering)?

Yes, there's a way!

As previously mentioned, you can tap into Isotope's internal properties and settings with .data('isotope'). For this issue, we are interested in getting the items within Isotope that correlate to what the user sees. There are two jQuery objects in the properties $allAtoms and $filteredAtoms. As you can guess, $allAtoms are all the items that Isotope affects, while $filteredAtoms are the items that have been filtered and/or sorted.

var $sortedItems = $container.data('isotope').$filteredAtoms;

Once you have that jQuery object, you can use it just like any other. For this example, I am highlighting the appropriate first and last item after a sort.

View Isotope - target first / last item sorting on jsFiddle