Andrea Leopardi, an Elixir core team member, writes about Elixir, Erlang/BEAM, networking, libraries and testing.
HN cites his posts on retiring the short_maps library over clarity regrets and on using NIFs to run native code from Elixir.
It is possible to start external processes from BEAM and interact with them. I've blogged a bit about it at http://theerlangelist.com/article/outside_elixir
You can also write NIFs (native implemented functions) which run in BEAM process (see http://andrealeopardi.com/posts/using-c-from-elixir-with-nifs/). The latter option should be the last resort though, because it can violate safety guarantees of BEAM, in particular fault-tolerance and fair scheduling.
So using BEAM facing language as a "controller plane" while resorting to other languages in special cases is definitely a viable option.
This was done previously with the `short_maps` library, which the author—a member of the Elixir core team—eventually retired and archived due to regrets from the undesirable impacts on clarity.
https://github.com/whatyouhide/short_maps https://andrealeopardi.com/posts/a-story-of-regret-and-retiring-a-library-from-hex/