You can exeecute PL/SQL blocks in sql*plus you just can't use SQL*Plus specific commands in it.
UTL_FILE isn't that difficult and probably the correct way to go for your needs. Once you get the PL/SQL written you can easily create a stored procedure and even schedule it to run at a specific time.
Check out:
http://psoug.org/snippet.htm/UTL_FILE_file_write_to_file_example_538.htm