If you were in a Standalone, you could customize the mail template vm files at $JIRA_INSTALL/atlassian-jira/WEB-INF/classes/templates/email directory. You would find vm files for each event in JIRA and one folder for text emails and another folder for html emails.
You can find more information at Customizing Email Content page.
Since you are in a OnDemand, I believe the plugin will be the best approach.
It seems this can be done, however it might need some java coding modification as we can see in this other quastion answer: https://answers.atlassian.com/questions/133722/how-do-i-customize-the-subject-line-for-notification-emails
The link you provided describes how to do it in confluence, as far as i know we have only jira on demand. Howerver, i tried to find the email templates but I don't know where to look for. In the article you provided there is a path that seems to point to a directory (/confluence/WEB-INF/atlassian-bundled-plugins/confluence-email-resources-x.x.jar) I tried to find a similar path for jira-on-demand but I didn't come up with anything.
I would apperciate your help
It's not possible to edit these in OnDemand as it requires file system access, and is a restricted function - https://confluence.atlassian.com/display/AOD/Restricted+Functions+in+Atlassian+OnDemand. There is an existing feature request to create a web-based editor at https://jira.atlassian.com/browse/JRA-7266.
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