Looking at the REST API documentation, I'm trying to figure out a way to take the XML of a report specification and create content (report) via the REST API? Can anybody think of a way to do this?

Not in the public API, but if you turn on network traffic tracking in your browser, when saving a new report in  CA, you can see that it sends a POST to http://<cognos-server>:9300/bi/v1/reports (or whatever you have setup as your CA gateway). If you capture this, you can look at the POST body, to see the format of what needs to be in there (along with the reportspec XML).

As far as public API - you could try a POST to the CONTENT endpoint (but you'd need to get all the properties right. If I remember correctly, the report specification XML is under the "spec" property.) 

https://www.ibm.com/docs/en/cognos-analytics/12.0.0?topic=api-rest-reference

regards,

   Jim