progress pictures:


The quick connects I decided to use is DB15. The 75 ohm and 0.1uF caps were all installed on the female SCART port. The 4PDT switch was installed with composite video down, RGB up. Composite video and audio left / audio right were fed into Video 3 / composite input.


End results / what I discovered:
Because I use mostly composite video for sync, if I don't feed 5V via SCART connector, the TV just thinks regular composite video is being fed in. This is a great A/B test as one can quickly see the difference in quality just by flicking the switch. This TV allows you to set color temperature, brightness, etc. All these pictures settings effect RGB! So set desirable settings in composite mode, and then switch to RGB to play.
Sega Genesis model 1 csync / sync booster SCART cable:


This system just shines in RGB on this TV. The best picture I've seen. Equivalent in every way to my 20" PVM.
Nintendo SNES mini with RGB amp:

Exact same color and brightness as composite video. Much cleaner picture.
NESRGB in my Top Loader (using exact same sync on composite video cable as the SNES):
This is where I run into trouble. This is what it looks in regular composite video from NESRGB:

Now same picture in RGB:

Yes, it actually does look "darker".
SMB3 world 1 level 1 shows the issue best:

Notice how much darker everything is. Also, the sky doesn't have uniform color. That white bloom is actually there! The other thing I noticed is "noise" in the picture. Diagonal lines of "noise" every few pixels apart, across the entire picture. Especially visible in the solid colors. Very hard to photograph, but if you look at the big "Mario Bros" title screen, you can clearly see it in the blue text.
This system looks and acts perfectly on my PVM .. I play it every day and never an issue.
2/13/2015: The issue is the cable. The diagonal lines are there in the SNES image above as well. When one uses CSYNC cables instead of sync on composite video, all the problems go away.