Glad you've mentioned it.
We have a plenty of open-source projects on several our GitHub hubs (the central one - intersystems, the russian one, and spanish one). But, unfortunately, nobody yet created msgpack implementation for Caché or Ensemble.
If you want to implement such component then we will republish it via those hubs with pleasure. :)
Looks like it's easy to implement something similar to Python-library, especially having current native JSON support in 2016.x