Sugar Sync & menu bar icons
There is no way to do this for all apps.  If it's possible, it's something the app developer would've had to choose to allow and so would only be possible on an app-by-app basis.  

free idea for those who want this bad enough to go make it themselves:

Create a library to inject into offending apps that makes -[NSStatusBar statusItemWithLength:] return nil.  That should probably be enough to prevent apps from making status items. (along with the usual disclaimer that modifying the behavior of programs and code that you do not control is usually a very bad idea)