After much searching around followed by a fair bit of trial and error, I figured out how to make use of Data View Parameters in a CAML Query in an XSLTListViewWebPart. Bear in mind I am configuring this using SharePoint designer.
So I have a list web part on a page, I open up that page using SharePoint Designer 2010 and begin editing under the hood. Once you've added a Data View Parameter you're then able to embed it as a parameter into your CAML query like so (Parameters in Bold):
<OrderBy> … </OrderBy>
<FieldRef Name="FileRef" />
Despite being so easy to implement, not once did I find it documented anywhere. I've labelled this post as I would have searched for it in the hope it helps out others looking for the same thing.