Encontré lo siguiente:
sql-statement, user, password, role and <connect-string> are string
expressions. When given directly, i.e. as literal strings, they must
be enclosed in single-quote characters.
Me imagino entonces que debería lucir mas o menos así...
Código SQL
[-]
execute statement 'select user_fullname from users where userid = :userid on external data source ''192.168.x.x:sched''
as user ''SYSDBA'' password ''masterkey'''
Me gusta más así:
Código SQL
[-]
execute statement 'select user_fullname from users where userid = ' || :userid || 'on external data source ''192.168.x.x:sched''
as user ''SYSDBA'' password ''masterkey'''
Saludos.