I would like to be able to display the related content (i.e. from a document library) in a task list that is tied to a 2013 workflow. Is this possible?
Let us clarify the question first
Tasks created in SharePoint 2013 from a workflow that uses the SharePoint 2013 workflow platform (not 2010) will not set the Related Content (aka WorkflowLink) field for the task in the associated workflow tasks list. I think only the approval process/workflow which uses the SharePoint 2010 workflow platform does this.
You want the user to be able to go to the workflow tasks list and view the tasks assigned to him, and for each task to have a link in the view that open the document for which the task was created.
The easiest way is for the user to open a task in the display form and she/he will be able to view the task details plus a field called Related Items that offers a link to the document.
If you really want to show the link to the document in the workflow tasks list view(s), then you could modify the CSR (Client Side Rendering) of the view using JS and JSLink property of the view webpart. See an example on how to do this here.
Be aware that this solution works only for tasks created by workflows that use the SharePoint 2013 workflow platform (not 2010 platform). Also, if you have multiple views, you will have to modify the CSR for each view.