Наверное есть:
Настройки - расширенные - содержимое - настройки для сайтов -
конкретный сайт - скрипты - папка для пользовательских файлов JavaScript.
Правда, пока не пойму, будет ли оно запускаться, если укажу на конкретную папку...
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Да, может
Цитата
User JavaScript is loaded and executed as if it were a part of the page that you visit. It is run immediately before the first script on the page.
Цитата
Opera is capable of running many Greasemonkey scripts. These are JavaScripts designed to work with the Greasemonkey enhancement for the Mozilla browser family. To tell Opera that a script file uses Greasemonkey notation, the name of the file containing the script must end with .user.js (all other .js files are assumed to be normal User JavaScripts).