You can do all sorts of things with pixelbender filters, the following link is an old experiment using a pixelbender shader to dynamically create a reflection. I think it throws a state error (its an old build) but just continue and it works the image can be moved around or 'swing'. Source code is attached.
Thanks alot! That should definitely get me started in the right direction. I'm very impressed with the things you can do with Pixel Bender. I never thought I'd see something like depth of field running in real time on my browser! I'm not exactly sure what kind of fullscreen transition effect I'll create yet, but from your code it looks like it's possible to capture the entire UI, which is what I was hoping for.
Fullscreen PB effects are certainly not recommended as Pixel Bender is CPU bound (software rendered)...but feel free to give it a go.