The PC Transporter provides composite out via a cable run to the internal video header of the Apple II.
This could be a missconception. The Apple II does not have a Video-In. The 4 pin connector right of Slot 7 provides the NTSC signal and is in general meant to augment it, like with a PAL converter for European TV.
Your intention is simply to use the Apple's RCA connector, so the user doesn't have to switch connectors. In general I would recomend to route the Apples signal from the 4 pin connector over to your card and have an analogue switcher between the Apple II's signal and whatever it produces, to finally be output on a new connector. Much the same way most 80 columns cards for the Apple II+ work. This is safe and carries no chance to damage the II.
Now if you want to do it the PC-Transporter way, then the only clean way is with the Apple IIe and closing the X3 pad. This will connect the AN3 output to the video logic, disabling the signal, thus closing the output transistor. Now an external applied signal can be applied and will show up at the video jack. See here for a short description.
The Apple II/+ isn't prepared for this, so you need to throw a wire across two chips.
After doing the modification AN3 can switch video on and off.
Another, less elegant way would be to drive an all black picture and overwrite the Apple II signal.
This will of course need previous synchronisation (and resynchronisation) using the same pin. Not sure if the is as easy possible as you seam to expect.
Either way will require careful design to not damage the II's output circuitry. Also, neither solution will work with the IIgs.