Added the following to the OP:
Another option for getting scanlines:
-set video output resolution to 1080 or native res and set retroarch render res to the same
-use the “scanline” overlay preset. In RGUI, go to settings-> overlay settings -> overlay preset -> wii -> scanlines.cfg
-go to shader options under options -> shader options -> shader passes and set to “1”.
-under shader 0, select “sharp-bilinear.glsl”
-under shader 0 filter, select “linear”
-leave shader 0 scale at “don’t care”
-select “apply shader changes”
You can adjust the scanline darkness by adjusting overlay opacity. I like darker scanlines so I set opacity to “1.00” This does make the picture darker, so I also had to crank up the backlight on my display.
I gotta say, using the scanline overlay actually looks pretty good, and has the advantage of running in 1080p without slowdown. It seems to have slightly less input lag than using shaders as well. It looks pretty comparable to using an XRGB-mini.