It is possible to connect to a database, not via an alias in the TNSnames file, but by writing the entire connect string, e.g. to SQL*PLUS:

sqlplus scott/tiger@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=localhost)(PORT=1521))(CONNECT_DATA=(SID=oracle)))

Is it possible to connect to a database by this method via the normal PLD logon (or another way), or does it require that all connection strings are present in the TNSnames file?

I know this is a bit exotic, but I just wondered

Yes, without any problem.

OK thanks, I missed the fact that the Database dropdown was a writeable combo-dropdown.

You can always write a whole connect string in a first field (username) on Logon dialog.