'Show Package Contents' in new iWork (Yosemite) File Formats
I've managed to expand the file by using the system (or any other) archive utility.

Here's how I got into the file:


Make a copy of the .key, .pages, or .numbers document
Change the file extension to .zip
Unarchive as you would any other .zip file to produce a folder
This folder is similar to previous show package contents bundle representations of older file versions
Make the changes you need to the document's data — in my case, this was modifying a few picture files that were used as icons throughout the multi-slide presentation
Re-compress the folder
Change the resultant zip file's extension to .key or .pages or .numbers