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I assume the entries that do not have a download button have the download hosted on another server ("home page"). What about linking the download button to that URL? Or if that's against user expectations, ask for an extra download page URL and link the button to that?
If plugin has no binary uploaded, pressing Download button takes user to plugin's website. See for example here: http://plugins.netbeans.org/plugin/34204 Implemented in Plugin Portal 2.0.