I know I'm a bit late to the party, but I just modified an Atari C-100 to output composite.
If you remove the battery casing temporarly and add a few wires you can lead the wires through the holes in the shielding and the rectangular cutout in the case and add a switch to maintain the original state without modifying the case. Switch can be glued to the compartment at the bottom with hot glue.
Quality between both is comparable, TV out has slightly more noise + grain and some interference from the speaker, composite has bleeding edges on the left (the ghosting on both photos is due to my smartphone camera). Adjusting the value of the pullup resistor at 1KOhm or using a different transistor for amplifying might help with composite. This modification is provided as "works for me" and was done on a "Tele-Games Pong" (is there such a thing as a PAL variant?).
It might also be possible to tap a usable signal from the CC3659 chip directly (one of the pins looked like a weak inverted video signal).
Schematic (MSPaint power!):
My modifications (excuse the mess, these were taking during 'development'):
Underside of console (please ignore my mediocre hotglue-fu):
Edited by Shockwav3, Thu Aug 16, 2018 7:46 AM.