It really depends on what you want the widgets to do. If the applications are backed by a database you can always query the database (assuming you have access) to get any pertinent data you may need.
The Applications may have API's that can be used to make your widgets. Accessing the API (if applicable) will allow you as much or as little outward customization as allowed.
I don't really think what you write you dashboard in matters too much as long as it can accomplish either or both of the two ways mentioned above.
As an example Wordpress can use XML-RPC to post to blogs. You could post to multiple blogs with one submission using that technology (though technically not part of the API I believe).
Without specifics on the Applications you are trying to use in your dashboard this is the furthest I can take this answer right now.