I’ve not seen any customers do that,  but that doesn’t mean it can’t be done.  We have been pretty successful getting custom capabilities of this sort to work via page includes with parameters passed from the Skuid page into the page include or iFrame.  An example of this is an answer Zach posted about Visual Flow 

We have wrapped the standard Chatter interface that works as a skuid page include.  The document uploading and manipulation features from Chatter work very well within Skuid.  But what doesn’t seem viable for you is that the preview is a modal popup - inactivating the rest of the page.  It seems you want to open a document preview and still have a set of fields available so the user can transcribe data from the document. 

What document viewer are you using?   

We aren’t using a document viewer within Saleforce yet, but there are two out there that we are looking at. Doument Viewer - Allows users to annotate and save on a document using HTML 5. or Attachment Viewer - Not able to edit the document, but can view it within Salesforce without downloading. The reason why we would use a document viewer is because the native Salesforce attachments can only be viewed without downloading if it is a PDF. We use both PDFs and other documents types.