There isn’t a way to do this using the Open XML SDK directly, but it is not hard.
One thing to review is how revision markup is put into the main document part. Check out screen-cast #8 in the following series:
Introduction to WordprocessingML
There is a DocumentBuilder example that shreds a document into sections. See the second example in the DocumentBuilder02.cs example, in Open-Xml-PowerTools. This might be an easy solution to your issue – first shred the document into sections, and then open the desired shredded document, then retrieve the text.