Well I am a little confused what you mean by load.
If you have a document that references plug-ins that you want to not be loaded you can remove the plug-in from the plug-ins folder and then it will not be found.
You might want to write a feature request to add a 'disable plug-in' switch whereby, when set, would cause Metro to act as if the plug-in was not found.
If you just want to select which plug-ins you see in the graphic editor, tracks window and efx window popups you can individually select 'Include in plug-in popups' for each plug-in in the plug-ins window by right-clicking the plug-in. Also Metro shows the most recently instantiated plug-ins in the recent menu as seen in the graphic editor, tracks and efx windows.