I work for a company that focuses on electronics design. We do hundreds of schematic reviews and board layout reviews every month. Currently, we do not have a way of reviewing board layout, except having someone open up the editing software and stepping through the design. However, an ultimate feature would be if collaborator natively understood KiCAD file formats.
KiCAD is a premier, open source, electronics design suite and the PCB files are plain text and designed to work with git. If you could consume kicad files and diff them for us; that would open up an entire new market for you.