Hey, Spelly is one of the few extensions built on internal versions of
the APIs, so it can do some stuff you can't with the external APIs. As
Lars said at I/O, there's currently a small gap between what's exposed
by the internal and external APIs, which we're working to close.
Still, I think you could approximate Spelly-like functionality by
having a robot insert a small gadget alongside a matching word. The
gadget could drop down the list of suggestions and be removed by the
robot after the user picked one.