Hi,
This is currently not supported. The reason is mainly that both casc and lookup merge duplicate entries and therefor cannot give you a link to one specific item. I might be able to add support for writing the ID of the selected item to a separate field and use this to construct a link in DispForm, but currently your best option would be to use code in DispForm to build a query based on your selections and get the item directly from the lookup list based on that query.
I can help you with a code snippet if you tell me some more details about your field names and the configuration for the lookup or casc.
Alexander