Provided that I have easy access to the main page's HTML, your statement on how easy this should be is fairly accurate.
Normally, I retrieve the page in question via SCP, SFTP, or if naught else is available, FTP, and make a backup copy of the document.
I ask where on the page you would like to see the button, and I insert the code in that area, visually checking for positioning in Google Chrome, Internet Explorer 10, Mozilla Firefox, and two versions of Opera.
If this interests you, feel free to reach out to me, and let's discuss.