As far as I know, you have 2 possibilities - implement TCP server in the Core itself, or install local Spark Cloud server and use it to pass messages to the core.
Example of the TCP server is in the documentation.
If you want to use standard Cloud messages, you will need to have local Cloud server to handle messages (and pass them to the Spark Core). Of course, if the local cloud server is not connected to internet, messages sent to server must be also originated in your local network.
See this thread for instructions for installing Spark Cloud Server:
Also, without internet access, you will need to use alternate solution for flashing firmware, but I guess you are aware of that.