I tried your code both in a user control loaded in the tooltip maanger, I also loaded the user control in a regular update panel (which is what the tooltip manager actually does), I even tried simply outputting it to the main page, even with the script file included in the head section. None of these approaches seemed to result in a google plus one button being rendered on the page, so I believe that this issue stems from the way this button should be used.
Once you are able to create this button in a user control, loaded dynamically in a regular update panel the same approach should work for the RadToolTip as well.
You can see a screen capture from my tests here: http://screencast.com/t/nFNyDHoNJ
and I am attaching the page and the UC I tested with as a reference. As you can see all three approaches have the same result. I also based my test upon the default demo of the RadToolTip, so you should find it close enough to your actual application.
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