You could achieve this with a new (or modified File) input type variable.
Expanding a directory in the MODX Browser triggers a number of events you can hook into - by default this is the listener that is added to the MODX browser for file tvs:
listeners: { 'select': { fn:MODx.fireResourceFormChange, scope:this}}
The "select" event seems to be triggered when you click "OK".
The "selectionchange" event is triggered when selecting both an image and a directory, so you could hook up an event to that to enable the select button.
I’d go for the feature request - an input option that decides if you can select files or directories would probably be a good idea if you ask me