It's possible, but not as simple as you would want. The location of the thumbnail varies depending on the product type, and also, the locations are not shared to IIS. (XMPie uses an image proxy to deliver the images rather than direct links.) So, you end up having to either expose the thumbnail folders to the internet via IIS or write a similar proxy/handler.
Anyhow...
From the orderproduct table, you can get the productID and join to the doc table. From the doc table, use the DocTypeId to get the document type from the docType table.
Now you know what type of document is ordered, the location to get the thumbnails from:
1) static: X:\XMPie\uStore\App\uStoreShared\ProductImages\Store_XXX\Product_XXXX\Pages\Thumbnails
2) dynamic: from the proof table, use the orderproductid to get the cache location and file extension. The path will be X:\XMPie\uStore\App\uStoreShared\ + cacheLocation + '.' + fileExtension
If you are delivering pdf proofs, then you will have to use the default thumbnail for the product instead of the customized proof - unless you go the proxy route and convert it.
Personally, I think XMPie needs to add it to the default order confirmation template, but this feature request is not high on the list at the moment.