In the sense that they run under CLisp, SBCL, Clojure, Scheme, Ruby, Python, the JVM and Javascript? I can't think of any that are portable in that sense.
Definitely seems to be -- they claim it's been used by a lot of different people (including, notably, the now-famous indie game Papers, Please) and is somewhere deep into version 3.something -- it's been around for quite some time.
From[0], the answer appears to be yes, but it greatly depends on the host. One can write Common Lisp code by using uppercase. Using $native will execute the following code as Scheme code. For Clojure, it looks like Shen provides a FFI for basic interactions.
