Selective hopper minecart unloading
My understanding, is you want a way to unload a hopper minecart, and leave it with some wheat in it, so that it will only pick up wheat on it's next trip.
This is a sketch of a solution.
- Put four pieces of cobble into the hopper minecart.
- Have your unloading station contain an item sorter accepting only wheat.
- Once the minecart is completely unloaded of wheat, as it departs, you can force a single wheat into the empty slot, to ensure only wheat gets loaded on the outbound trip.