To mod a Genesis with a CXA1145 chip you can just build two of these amps for the luma and chroma lines and you're set!
S-Video mod (for all systems using the CXA1145 or MB3514)
Here's all the info you need for the S-video mod. This will work Neo Geo AES systems as well as any other console that uses the chroma encoders CXA1145 or MB3514. I find the output was a little better when both signals had 33ohm resistors at the output. Play around with these values until you find what's suitable for you.
The MB3514 exists in TSOP form factor and can be found on the Model 2 of the Genesis. I have three Mega Drive 2 (Asian PAL Edition Model MK-1631-07) and they all have the MB3514 video encoder.
According to the MB3514 datasheet, the MB3514 can directly drive S-video output. Just add a resistor and coupling capacitor to each of the chroma-out and luma-out pins on the chip. I am able to get a nice video output by adding two 75 ohms resistors and two 220 uF capacitors. This S-video mod is similar to the S-video mod described for the genesis 3.