Alrighty. I just added a feature to the Matrix layout to add thumbnails for the previous/next buttons. Get the latest code and check it out.
The way it works is that now there's a new data type that you can ask GalleryLayout::loadLayoutTemplate() for, called 'peerThumbnails'. This gives you the information you need to get a thumbnail for any of the peers.
In the matrix layout, I changed around the way that the navigator works a little bit so the first/back/next/last peers now all have a url and an item. Then in itemNavigator.tpl there are new blocks like this:
{g->image item=$layout.navigator.first.item image=$layout.navigator.first.item.thumbnail maxSize=40 class="giThumbImage"}
This drops in a thumbnail for the "<< first" item. I did it for all of them. It's not pretty, yet -- but it works. Let me know what you think.