I have wondered this myself. You may be able to route the coil split wires through a push/pull switch on their way to the blade.That's the idea, anyway. I know enough to be dangerous, but not enough to know if something will work without experimenting or consulting someone more expert. If it does work, you'd have:
Push/pull down (regular positions)
- Position 1: (down) Bridge Humbucker
- Position 2: Bridge Humbucker with Neck Singlecoil
- Position 3: Bridge and Neck Humbuckers
- Position 4: Neck Singlecoil with Bridge Singlecoil
- Position 5: (up) Neck Humbucker
Push/pull up would be:
- Position 1: (down) Bridge Singlecoil
- Position 2: Bridge Singlecoil with Neck Singlecoil
- Position 3: Bridge and Neck Singlecoils
- Position 4: Neck Singlecoil with Bridge Singlecoil
- Position 5: (up) Neck Singlecoil
Positions 2-4 would all be both singles, but you'd get the two extra options of split bridge or split neck.