Not sure if this is what you hoping for, but if your DVR is an ONVIF compliant NVR (most security systems are) the CamController.com software just got support for NVRs. It can help to expose the rtsp addresses for the cameras. You can then add the cameras to OBS as a media source. Uncheck the "Local File" box in the media source and you can add the RTSP address.
We use CamController.com with a variety of different cameras for live broadcast this way, you can also setup hotkeys in OBS to change scenes from CamController.