Add button in Ribbon dynamically

How can I add button to an existing tab "Project" in Ribbon?

I can't  use elements.xml file, because that would show that button on every page and I need it on only ONE page.

My guess is that I need to build new web part and add it to page, and with javascript insert button to existing tab section (Workflow, for example) ? But how?

I am trying to do it with this code, but it doesn't work.

http://msdn.microsoft.com/en-us/library/ff407578.aspx

I don't need new Contextual tab, only put one button to an existing tab "Project". Can somebody tell me what do I need to change in order to make this work?