Here is what I received from senior management last year on the issue:
We are aware of the "seat block" product desired by customers, and the fact that how it works by design today isn't always ideal. As you mention, when we have looked at it, there are some complicated implications within our Departure Control System that make this a bit more complicated than it looks at first glance. That being said, it's not an impossibility, it just needs to be slotted in within our laundry list of product enhancements.