I just updated my copy of mylyn to the current version and I was happy to find that it now works easily with Google Code. In order to set it up, all you have to do is add a repository for the Google Code issue page. The easiest way to do this is to select the sample server entry for Google Code when adding the repository. All the necessary fields are filled in for you to make it work. Generally all you have to do is change the url is puts in the server field to point to your specific application. You don't get the fancy edit page that you would with something like Jira, but it will pull up the appropriate web page in eclipse's web browser. Pretty cool.
Also, if you hate that mylyn removes a file from context when you close it, there is a new check box in the settings that allow you to change that behavior.
Mylyn tends to be a little buggy out of the gate, but it eventually starts working as it should for me. Sometimes I have to move around the interface a few times to get things rolling after an update.