Is it possible to make a node marked as "expandable" if the model does not (yet) have any data loaded on that node?

What I'd like to do is set up a signal/slot connection to the view's expand() signal to tell the model to look up data for a given node.

I'd like to do this because looking up all the data at once takes several seconds, so taking in pieces of the data as needed should take significantly less time.

Currently, if there's no data under a node, there is no '+' symbol to click on. Can this behaviour be changed?