Thank you for reading!

I have tried many variations of this and it successfully appears in web.assets_backend.js. However, no version overrides the Composer class field default of isLog. Thank you for your consideration.

Odoo file: mail/static/src/models/composer/composer.js  line 1214:

/**
* If true composer will log a note, else a comment will be posted.
*/
isLog: attr({
    default: false,
}),

One of many override attempts:  

odoo.define('my_mail_module/static/src/js/my_mail_composer.js', ['mail.Composer'], function(require) {
    "use strict";

    const Composer = require('mail.Composer');
    const { attr } = require('mail/static/src/model/model_field.js');

    Composer.include({
        isLog: attr({
            default: true,
        }),
    });
});