Judging from this post there is an eight steam version of Musepack in beta testing (but the spec is not frozen yet).
What distinguishes it from previous versions? Now it is container-aware. Previous versions store just audio frame in the continuous bitstream with no defined behavior on seeking nor demuxing. It was almost as fun as Monkey’s Audio container.
Now Musepack has a chance to spread in the wild (i.e. in other containers than .MPC). That mostly depends if there will be standard way to store them in the other containers (that’s where Ogg Vorbis failed). Well, good luck.