A block that you can put scripts into, they could be scratch scripts, or some other type. This would have to be for more advanced users. Maybe there could be an "advanced" button, and once you click it you can do custom blocks.
There are a couple of modifications based on Scratch that have this feature built in, notably BYOB. It's free for download here: http://scratch.mit.edu/forums/viewtopic.php?id=10887 . Enjoy