SQL*Plus – Connecting to Oracle Without a Listener

 sqlplus user/pass@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(Host=hostname.network)(Port=1521))(CONNECT_DATA=(SID=remote_SID)))

Maybe, and this might be dependant on the command line environment you're using, you need to quote the string, something like

 sqlplus "user/pass@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(Host=hostname.network)(Port=1521))(CONNECT_DATA=(SID=remote_SID)))"

or

 sqlplus 'user/pass@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(Host=hostname.network)(Port=1521))(CONNECT_DATA=(SID=remote_SID)))'

So, with thanks to @YasirArsanukaev for the time he put in, I have found a solution which works, but which I can't really explain.

Riffing on the LOCAL_LISTENER thought, I was reading this other answer where it said:

The database uses the LOCAL_LISTENER parameter to identify the listener it should register with. By default that is null, which according to the documentation is equivalent to hostname:1521.

So I tried to ping my own hostname and couldn't - it looks like some IPv6 problem, receiving a general failure message.

So I took the advice from that answer

SQL> alter system set LOCAL_LISTENER='(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521))' scope=both;
SQL> alter system register;

and it now works, presumably because it can resolve the localhost reference, where it was failing the resolve the actual hostname.