@plainbroke You will need a external tool like ffmpeg or vlc to convert the stream.
I wrote a small wrapper that calls vlc with the necessary parameters. You can find it here.
There are other tutorials using a docker containter, too. For me this one was the most effective one.
Be aware that the conversion cost a lot of cpu power. Some users had problems running the converter on the same Pi as the mirror is running on.
You then can embed the Stream with this module using the setting embedElementType: "img" or you can use my MMM-SynologySurveillance which supports mjpeg since the last version.