Thanks for reaching out to us! Unfortunately currently there isn’t a way to do it. The “Reverse Effect” checkbox on the Effect Painting popup doesn’t reverse the entire tilt-shift but instead makes it so wherever you paint (i.e. drag over the photo with your mouse button down) the tilt-shift will come in, and everywhere else will be normal.
We will look at including this functionality in a future Ribbet update. Let us know if you have any other questions.
Regards – The Ribbet Team