SAThreadSearch.user.js

// ==UserScript==
// @name         SAThreadSearch
// @namespace    forums.somethingawful.com
// @version      0.2
// @description  Inline thread search
// @author       Me
// @match        http://forums.somethingawful.com/showthread.php*
// @match        https://forums.somethingawful.com/showthread.php*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    $('<form action="query.php" method="post" class="threadsearch">')
        .append('&nbsp;<input id="searchbox" type="text" placeholder="Search thread" name="q" value="" size="25">&nbsp;')
        .append('<input type="submit" class="bginput" name="Submit" value="GO">')
        .append('<input type="hidden" name="action" value="query">')
        .submit(function(event) {
            $('#searchbox').attr('value', function(i, val) {
                val = val.replace(/threadid:\d+\s+/g, '');
                if (val === "") { event.preventDefault(); return; }
                return 'threadid:' + $('#something_awful').data('thread') + ' ' + val;
            });
        }).insertAfter($('.top .thread_bookmark'));
})();