Hi,
For those that are looking for a way to use gstreamer: it turns out the gstreamer pipeline just needed another element to process the stream to be able to display it. This pipeline does the trick:
gst-launch-1.0 rtspsrc location=rtsp://192.168.0.184:554 ! rtpjpegdepay ! jpegparse ! jpegdec ! autovideosink sync=false
Recompiling the firmware to output 420 is not required, because it also works with the original firmware after I reflashed the RT1062.