This is something on our to-do and wish-lists.
We attempted it once, in the middle of a season, a few season back. It didn't get the response we had hoped for (which was a pity), but that's not the main reason for it not being in the league at present.
Under the current league setup with volunteers, software and so forth, it's an administrative nightmare. Changing details with horse files, and changes to the TOM DB, etc. That's why we now allow two weeks for the D&D process. One week in the middle of the season wouldn't be enough, and I'm not sure everyone would want to take a two week break mid season. You might start to suffer from withdrawal symptoms
But, it is something we are working on, and if we can get a more feasible method for doing it, I don't see why we can't re-introduce it.
We would also need to increase a trainers welcome dollars, so they could compete with a richer trainer, who just didn't do so well the previous season.