The solution is VLC
with
vlc mmsh://212.12.127.212:8080 --sout file/:/tmp/harddisk/media/tmp.avi
I'm saving a live TV stream to a file.
With mediatomb I'm serving this file to the DSM-520
Known problems
1) The stream stops after a while with
nothing to play
2) There is no sound (alternatively there is sound but no picture).
3) I don't know how to stream from a DVB-T stick (instead of an internet location).
Does anyone have a solution?