Yes, Slicer’s python is embedded.
subprocess.call should generally work, however, it will block Slicer (GUI will freeze) until the call completes (you can use Popen or Slicer’s CLI infrastructure instead for asynchronous calls). I just tried subprocess.call(['sleep', '10']) on linux with a recent nightly, and it worked fine.
Please show a minimal example if you can – maybe someone will notice a problem.