Hmm, you mean by using a hidden variable defined in the same POST block as the "Discard" button? Not sure that this is possible. Check the admin template that is used in the same discard operation as the one you're looking for; perhaps you'll find an answer to your question there. You could also peek at the code (in /kernel/content/).
That works well for redirecting to a static location, thanks for the tip. (Watch out though - don't try passing the link you're redirecting to through |ezurl - that doesn't work.)
Now, does anyone know how I can set that dynamically to the previous page, i.e. what template code will give me the url of the page the user was on last? Is this even possible? Returning the user to the most recent page on cancel does seem to be obvious functionality, but I can't work out how to do it!