I'm certainly no expert here, so take my thoughts with a large, large block of salt. By way of example, I do know that the typical action of dragging a file on

I'm certainly no expert here, so take my thoughts with a large, large block of salt. By way of example, I do know that the typical action of dragging a file on to the application's icon in the dock will allow an app to open that file, just as would the open file dialog. Not allowing this would be, obviously, unworkable. Similarly, I believe (although I don't know) that a key-combo associated with that app (as is usually used to invoke screenshots) would signal that the user is acting on the application and, hence, authorizing the action. Although, I'm not sure screenshots would be inhibited by the sandboxing to begin with...