gm-rmc-test-1.user.js

// ==UserScript==
// @name        GM_rmc Test 1
// @namespace   https://github.com/arantius
// @include     http*
// @version     1
// @grant       GM_registerMenuCommand
// @run-at      document-start
// ==/UserScript==

dump('GM_rmc Test 1\n');
GM_registerMenuCommand(
    'GM_rmc Test, http, -start', function(){ alert('OK!'); }
    );

gm-rmc-test-2.user.js

// ==UserScript==
// @name        GM_rmc Test 2
// @namespace   https://github.com/arantius
// @include     http*
// @version     1
// @grant       GM_registerMenuCommand
// @run-at      document-end
// ==/UserScript==

dump('GM_rmc Test 2\n');
GM_registerMenuCommand(
    'GM_rmc Test, http, -end', function(){ alert('OK!'); }
    );

gm-rmc-test-3.user.js

// ==UserScript==
// @name        GM_rmc Test 3
// @namespace   https://github.com/arantius
// @include     about:blank
// @version     1
// @grant       GM_registerMenuCommand
// @run-at      document-start
// ==/UserScript==

dump('GM_rmc Test 3\n');
GM_registerMenuCommand(
    'GM_rmc Test, about:blank, -start', function(){ alert('OK!'); }
    );

gm-rmc-test-4.user.js

// ==UserScript==
// @name        GM_rmc Test 4
// @namespace   https://github.com/arantius
// @include     about:blank
// @version     1
// @grant       GM_registerMenuCommand
// @run-at      document-end
// ==/UserScript==

dump('GM_rmc Test 4\n');
GM_registerMenuCommand(
    'GM_rmc Test, about:blank, -end', function(){ alert('OK!'); }
    );