Well, if a primary objective is to protect simple-minded users from the complexities of the shell, you really should avoid using angle brackets on the command line altogether.

To put that another way, if users don't want to be bothered with the standard usage of angle brackets, things will go much better for all concerned if you simply forbid any use of angle brackets.

Why? Because when you forget to use quotes in OP approach to appending, you can get in bad trouble. REALLY Bad Trouble. As in, deleting the very data you were supposed to preserve by appending.

So do it like this:

  Usage:  script_name  [-a]  output_file_name
       by default, output_file is created as a new file
       use "-a" to have output appended to existing output_file
[download]

That is, use "-a" instead of an angle bracket. Or better yet (assuming that it's more important to avoid deleting existing data), make appending the default mode of operation, and have an option "-n" (for "new") or "-b" (for "begin" or "blank-slate") or whatever, to create/truncate the output file.

Rest assured, the OP design is the farthest thing from "user friendly". You have to do better than that.