The issue with processing E-6 in obscure sizes is that the equipment (hangars) that hold the film probably don't exist. I suppose one could put a 4x10 sheet in an 8x10 hangar, but it would probably come out in the first developer. The hangars would have to be modified with some additional film clips or restraints. Have you considered shooting your E-6 as full 8x10 sheets with a 1/2 dark slide in your 8x10 camera, then cutting the transparencies in half after processing to make 2 4x10's?