Actually it is so easy that I could do it myself... Hold on, I'll try to fetch my rough but working code.
'warn_on_new_replies' => array(false, 'profile', 'edit_profile'),
<input type="hidden" name="default_options[no_new_reply_warning]" value="0" />
<label for="no_new_reply_warning"><input type="checkbox" name="default_options[no_new_reply_warning]" id="no_new_reply_warning" value="1"', !empty($context['member']['options']['no_new_reply_warning']) ? ' checked="checked"' : '', ' class="input_check" /> ', $txt['no_new_reply_warning'], '</label>
I don't think I need to add the language string, you know it WAY better than me. Still... Yeah there is no default function, I really have to stop considering stuff as default even if they aren't... Curious that my own SMF looks a lot like Elk, functionality-wise, with a few additions and some removals.
Sorry for the late reply, as you probably know I was on holidays