Thanks, Willi, for the suggestion. But I couldn't get it to work inside an Action. I recorded Save Selection as part of an action, but of course, it saves the selection to that particular file. When the Action is run on a folder full of images, the Action comes to a halt because the saved selection is only in a certain image, and the Action doesn't know how to access it in an image that is not currently being processed.
I doubt whether a PS Action can save a selection as part of the action. Probably an oversight on the programmers part. The closest I could get was to record an Insert Menu Item, the menu item being Select All.
The interesting thing is: the scenario works when recording the Action. I set the selection, then start recording my actions – the crop and save and whatever – and it works. The image is cropped and saved. But when played back as an Action, the settings for that particular selection are not stored in the Action. And they should be. It would be quite easy to record the co-ordinates of the selection as part of the Action (X-Y co-ordinates of the four corners), but PS hasn't been set up to do it – as far as I can tell. The problem would be: the selection would only be valid for images of exactly the same size.