I think when it comes to ice cream cakes you have to accept your limitations. There are some things you just can't do with them. Also, considering the fact that you really can't display them for very long outside of a frozen environment leads me to the conclusion that decorating an ice cream cake to the same extent as a cake that can be left out and displayed before being eaten, isn't the best use of time or resources. Decorate it simply using methods already deemed successful and make it look appetizing, which really is the whole point of decorating a cake anyway. Your primary concern should be, "does it sell?"