Where HOSTNAME I would like to use my systems hostname, from environment variable $HOSTNAME. Is it possible to specify it using: (System/getenv “HOSTNAME”) or similar?
No, there is no support for anything dynamic in EDN beyond a couple of built-in tagged literal readers (#inst and #uuid I believe).
It seems like it would be a nice enhancement for tools like nrepl.cmdline to accept flags that indicated values should be taken from the environment. At work we have a wrapper for starting REPLs so that we can get the port from somewhere other than the deps.edn:main-opts.