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A direct excerpt from the comments in the source-code (3.1.0) resolves my question:

(https://github.com/opencv/opencv/blob/3.1.0/modules/videoio/src/cap_gstreamer.cpp)

 * The 'filename' parameter is not limited to filesystem paths, and may be one of the following:
 *
 *  - a normal filesystem path:
 *        e.g. video.avi or /path/to/video.avi or C:\\video.avi
 *  - an uri:
 *        e.g. file:///path/to/video.avi or rtsp:///path/to/stream.asf
 *  - a gstreamer pipeline description:
 *        e.g. videotestsrc ! videoconvert ! appsink
 *        the appsink name should be either 'appsink0' (the default) or 'opencvsink'
 *
 *  When dealing with a file, CvCapture_GStreamer will not drop frames if the grabbing interval
 *  larger than the framerate period. (Unlike the uri or manual pipeline description, which assume
 *  a live source)
 *
 *  The pipeline will only be started whenever the first frame is grabbed. Setting pipeline properties
 *  is really slow if we need to restart the pipeline over and over again.

A little bit further down, the following capabilities are set (For GStreamer 1.x) :

caps = gst_caps_from_string("video/x-raw, format=(string){BGR, GRAY8}; video/x-bayer,format=(string){rggb,bggr,grbg,gbrg}");

Which is what the appsink/opencvsink is capable of.