I have a workaround for this - it isn't pretty but it seems like it will do the trick. I can't directly manipulate the leader as desired but I can get the result that I want.
When calling InsertSketchBlockInstance, the insertion point argument (a mathpoint) determines where the sketchblockinstance is inserted. The instance is essentially just a copy of the sketchblockdefinition. When the sketchblockinstance is placed, the leader geometry is instantiated (regardless of whether or not it is visible). After insertion, although the leader cannot be manipulated, the position of the sketchblockinstance can be. What's more, when the sketchblockinstance is moved, the pointer stays put. In this way, I can insert the blocks where I want them to point, and then move them to where I want them to be located.
I can't seem to get a predictable behavior out of the leader as it is saved. I'm not certain whether it is part of the definition or not - when I manipulate an instance
Please let me know if you are aware of a more direct method of manipulating sketchblock leaders using the 2012 API.