I suspect the reason this hadn't already been implemented is because the Javascript programming pass was largely done to replicate previous features more smoothly, rather than using that functionality to add more features.
This will probably end up implemented if there's a good example available Mojko and ultramage can copy code from, otherwise breaking down the deck builder interface and rebuilding it for drag-and-drop will be too much effort.
(Unless someone can donate that effort, but the last person to indicate willingness had other interests.)