Wow, that's way more complicated than what I thought it was doing. I thought it was just doing a blanket resizing to ~1.5x normal size, and applying a very basic bilinear filter to the entire screen. At least that's what I remember nutki saying at the time, years ago, though that may have just been an oversimplification to break it down into something we could understand. In actuality I have no idea how bilinear filtering is done so perhaps it's exactly as you described it.
At the end of one of my videos long ago, I demonstrated Lameboy v0.9 a bit, which was the first version to implement the filtered full-screen (previous versions had full-screen but it was just a sheer nearest-neighbor resize with no filtering) which attempts to show the difference between the filtered and unfiltered fullscreen options --
https://www.youtube.com/watch?featu...D5zdyYuc&list=UUs3pZIh0PoWSt2TOkNuUF5Q#t=411s
Now if nutki upgraded the filtering algorithm between v0.9 and the final version, I would have no idea. If he did, it may be worth looking back at v0.9 if it has a simpler method.