You could create a widget that simply loaded pages into the dialog with AJAX. Doing it that way, you'd be able to allow users to create content and drop widgets just like when they're editing a page. (Because they will be!) :)
Also, I'd create a special section of pages where all the "dialogs" live, then they'll all be together.
To tie everything together, just place a page selector on the widget's designer and bam, they can select which "dialog" is displayed.