I have added an RGB input to my Toshiba MD13M1 13" TV/DVD Combo. It was really not that hard. I traced the RGB signals in reverse from the picture tube until I arrived at the video decoder chip which converts all the composite/s-video/etc. inputs to analog RGB output.
I spliced into these lines, adding a switch to retain original functionality. Basically you are hijacking the internal RGB signals and replacing them with your own, in my case from a PS2. I had to pull up the RGB lines from the PS2 with 5V (available on PS2 multi-out) and some resistors, experimentation was required to find a suitable pullup value to get acceptable brightness. I used a pot to make the brightness adjustable.
For sync I kinda cheated but it works great. I simply connected the composite video signal from the PS2 to the composite input on the TV, and let the onboard decoder chip do its thing like usual.
Operation is pretty simple. In the off position, the TV functions like normal. When I want to use the RGB input, I set the TV to the composite input, flip my switch, turn on the PS2 (set component output to RGB in the settings screen), and we have beautiful RGB. The only drawback is there is no on-screen display, and the brightness/contrast/etc. controls on the TV have no function. This is because I tap in after all of these adjustments are made.
I believe this can be done to any somewhat modern TV using this method, but you will have to do your homework. Finding a datasheet for your TV's video decoder chip is very helpful.
If anyone is interested I could take pictures or provide more info.