Thanks Gregory,
I should've mentioned that the background wasn't black. (It was a looping texture)
In the end i made 2 quads - one with the looping texture as a background, and the other with a feathered hole in the middle of the looping texture.
Then i set my rolling text to 'render priority normal' so it sandwiched between the layers.
Looks just like a feathered mask.
#Carbonite