Currently this can be done in FlexDash. The "super widget" takes the form of a SubflowPanel, which can only be placed into a grid. It cannot be placed into a regular panel, which would be desirable.
I tweaked the current panel layout machinery so it works pretty well to create such widget assemblies but eventually someone should come up with alternatives.
I haven't tried creating a package from a subflow with a subflow panel. The subflow panel is a config node that is associated with the subflow. I would assume that gets exported. It doesn't depend on the grid since it's subflow instance nodes that are "in the grid". Something to try...
Thanks for pointing out the package angle, I hadn't considered that at all!