I asked the CM at Starring Rolls for a slice of the Grapefruit Cake. I knew it wasn't in the display case. She went to the kitchen and got a slice and put it in a container. It rang up same as other cake selections. Give that a try, worked for me! And, Yumm-o!
We've had reports here of people ordering the grapefruit cake to go at the Brown Derby, but not in a long time. So I think it may be possible, just not probable. I would ask at Starring Rolls, that is, if you can tear your eyes away from the cupcakes....