Interesting project.
Don't know why I didn't think of using rgbadjust, certainly makes more sense.
If the artifacts that subtract sometimes has bothers you, try:
Code:
R=crop(x,0,0,-3,0).pointresize(breite*6,hoehe).
\pointresize(breite,hoehe).RGBadjust(1,0,0,1)
G=crop(x,1,0,-2,0).pointresize(breite*6,hoehe).
\pointresize(breite,hoehe).RGBadjust(0,1,0,1)
B=crop(x,2,0,-1,0).pointresize(breite*6,hoehe).
\pointresize(breite,hoehe).RGBadjust(0,0,1,1)
layer(r,g,"fast").layer(b,"add",85).rgbadjust(3,3,3,1)
Thats quite close to the subtract version, with a little tweaking of the final rgbadjust you might get a good match.
Edit: I suppose it is a trade off between subtract artifacts and the quantisation issues as you said.