I've created my custom object, I want to add a lookup or (ideally) a master detail relationship to ContentVersion. However when I go to create a New Relationship on the custom object, I don't see ContentVersion in the "Related To" drop down list. Is this a permissions issue, or is this just not possible? Any advice would be much appreciated, thanks!
ContentVersion can not be the parent of any custom object.
ContentVersion can be the child, via a lookup relationship to Custom or Standard objects. This is done by
Setup | Customize | Salesforce CRM Content | Fields | New
ContentVersion and its related objects
ContentWorkspace, ContentDocument etc. were acquired through an acquisition and behave without complete adherence to normal standard and custom objects. There is an Idea I did years ago https://success.salesforce.com/ideaview?id=08730000000BrYFAA0 related to some limitations.