1. You can load additional JavaScript file after the load of Ajax Toolkit. In the script you can assure the usage of "sforce" object in authorized context.
Please specify the file URL and create a bookmarklet to load it.
2. You can also specify your own Gist (snippet script hosting in GitHub) snippet.
To refer your script, please input your id of Gist and the script filename.
For example, if your gist url is "https://gist.github.com/1436661", input "1436661" as "Gist ID", and if the loading JavaScript filename is "toggle-language.js", put it in "Filename" field.