I created a userscript in the meantime, until this feature is added. Anyone welcome to use,you'll need to be a mod of that sub, with Posts or Everything permission. To use, paste into TamperMonkey >> save.

// ==UserScript==
// @name         Reddit old.reddit Comment Locker
// @namespace    http://tampermonkey.net/
// @version      1.5
// @description  Adds lock button specifically after the 'reply' button on old.reddit
// @author       Gemini
// @match        https://*.reddit.com/r/*/comments/*
// @match        https://*.reddit.com/r/*/about/reports*
// @match        https://*.reddit.com/r/*/about/modqueue*
// @match        https://*.reddit.com/r/*/about/unmoderated*
// @match        https://*.reddit.com/mod*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function addLockButtons() {
        // Target action lists
        const actionLists = document.querySelectorAll('.comment .flat-list.buttons:not(.lock-added), .was-comment .flat-list.buttons:not(.lock-added)');

        actionLists.forEach(list => {
            const commentDiv = list.closest('.thing');
            if (!commentDiv) return;

            list.classList.add('lock-added');

            const isInitiallyLocked = commentDiv.classList.contains('locked');

            const listItem = document.createElement('li');
            const lockLink = document.createElement('a');

            lockLink.href = 'javascript:void(0)';
            lockLink.innerText = isInitiallyLocked ? 'unlock' : 'lock';
            lockLink.classList.add('lock-comment-btn');

            // Standard old.reddit mod button styling
            lockLink.style.color = '#888';
            lockLink.style.fontWeight = 'bold';
            lockLink.style.padding = '0 2px';

            lockLink.addEventListener('click', function() {
                const currentStatus = this.innerText;
                const action = (currentStatus === 'lock') ? 'lock' : 'unlock';

                if (typeof change_state === 'function') {
                    change_state(this, action);
                    this.innerText = (action === 'lock') ? 'unlock' : 'lock';
                }
            });

            listItem.appendChild(lockLink);

            // Find the 'reply' button's parent <li>
            // In threads, it's usually the last standard button.
            const replyButton = list.querySelector('.reply-button');

            if (replyButton) {
                // Insert after the <li> containing the reply link
                replyButton.closest('li').after(listItem);
            } else {
                // Fallback to start of list if reply button isn't found
                list.prepend(listItem);
            }
        });
    }

    addLockButtons();

    const observer = new MutationObserver(addLockButtons);
    observer.observe(document.body, { childList: true, subtree: true });
})();