I have a functioning proof-of-concept implementation for the new scaler plugin system. It has some hacks for now to make it easier to prototype. Also it is missing GUI features and ignores configuration settings.
The benefits are really starting to show. Adding new plugins is really easy and takes no additional configuration from the sdl backend. Also, I have made small changes to the DotMatrix scaler to take advantage of the coordinates passed from the backend. Now it does not change the surrounding pixels when the mouse is moving.
If you want to try it out, my code is on github. "Ctrl alt +" and "ctrl alt -" increase and decrease the size (as usual) but "ctrl alt [" and "ctrl alt ]" cycle through the different scalers.