The ListDisplay control uses custom ControlDesigner which makes adding a new template in the requested way almost impossible
. You need to create a custom control designer and map Telerik.Lists.Design.ListsControlDesigner. Then inherit from ListDysplay control and create new enum simolar to ListDisplayMode enum. Here you need to add your custom mode. Afterwards you should override each methiod in the ControlDesigner and ListDysplay control where the ListDisplayMode enum is used and replace it with your custom one.
We will migrate the module to Generic Content impementation for 4.0 and this will siplify managing the designer.
the Telerik team