Fortunately I was just now able to make progress on the issue.
It turns out, the endpoint was requiring a content length header to be sent along. Postman sends those by default, so I wasn't seeing the issue there. I tried making the call using a curl request and I got the notification that a content length was needed. Even though I was using the dotlogger, I wasn't receiving that notification through the velocity script. Is there someplace in dotcms logs that I would be able to see this information? Or do they bury it?