This isn't possible, as the guest OS cannot get more memory allocated to itself than has been assigned to it. Let's say you have five guest machines that "share" all their memory through this new feature. If you gain access to one of the machines and completely rewrite all the memory it has inside it, you still have only doubled the memory usage as the host os allocates the new memory for the guest os. Rewriting in the same memory multiple times will not increase memory usage any more than that as it will still be writing in the same memory area the host os allocated. Even if the attacker was able to gain access to all five machines and rewrote their memories, the host os would still only have five times as much memory usage as in the beginning (which is pretty much the way it is now without KSM).
So worry not, this should not be exploitable. *knocks on wood*