With a bit of Javascript, and the ability for Canvas to have your own theme with custom CSS/Javascript, you can enabled this option for all instructors at your institution.
The following video shows how you to set 'Enable recording for this conference' to true (and prevent the instructor from disabling it). See
Here is the custom JavaScript snippet to upload to your theme (or add to any existing JavaScript you have in your theme).
/* Force conferences to be recorded */
$("button.new-conference-btn").live("click",
function() {
rec = $("#web_conference_user_settings_record")[0];
if (rec) { rec.checked = true; rec.disabled = true; }
}
);