Forcing a degrading composite signal, electrical gurus needed

I'm still working on my scart switch box, I've going through a 2nd design revision and I need to get something figured out.

The box will has this built in: http://www.amazon.com/BG-460-SCART-System-Digital-Converter/dp/B00988GMLG

This converter will accept rgb, composite and s-video. The problem is I can't get it to force to s-video. It mostly always chooses composite/av

I've gotten it to choose a s-video a few different ways. Reseting the plugged console on and off will force the converter to search for signals:
It will search in this order:

A/V NTSC
A/V PAL
SV NTSC
SV PAL

Once in a blue moon while doing the reset; s-video will pick up.

If the signal is totally gone long enough (1/2 second) from the convector box it throws up a "signal not found" once signal is applied it again, it starts searching in the above order.

I first tried a pot on the line to reduce the signal power hoping it will still "see" a signal but start searching. No dice on this one.

Next guess was forcing the signal through a sync striper. This sorta worked, but still very inconsistent.

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My ultimate goal here is to put a momentary switch on the box, that defaults to standard composite. When the button is pressed, it causes the convertor box to start searching for signals and you
release the button once it lands on SV NTSC.

OR

Figure out how to actually tell it that its an s-video connection.
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I can understand the boxes confusion, I'm sending it a full composite feed. Not just luma signal.

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So, is there a way to strip the "chroma" from the composite feed and hopefully this tells the box I'm using s-video.

OR

Cause a poor signal on the composite line to force it to search for signals going through the above list.
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Something interesting I notice. When I was trying to experiment earlier I fired on my 2600 for the first time, and the box kept cycling through the above list. It never got a "signal not found" but couldn't detect something it could work with. I reseated the cart on the 2600 and then it worked and obviously chose composite and not s-video.

Any help would be great!
Thanks