Should be easy to add if we know how it works. What I find strange about this feature is that there is already a 2D color offset register that adds an RGB value to an entire layer. It's used for fade-in/fade-out effects, such as Lost World's 'Sega' screen and the lightning flashes during the introductory sequence.
How do you think it ought to be added? I can give it a go. My source tree is in a state of disrepair as I am trying to incorporate a newer config system, and thus have had to touch a lot of files. But I can probably clean that up soon.