Got no schematics, but maybe this also helps:
Essentially you activate _XLCKEN and feed the external sync (extracted from composite) into _XCLK - this synchronizes the Amiga's output to your external source. Adding a PLL will make your Amiga survive a dis/reconnect of the external sync source (it won't break but it'll crash otherwise).
Then you use the _PIXELSW signal to switch between the external composite and the Amiga's (depending on the model you may have to generate that first). _PIXELSW is active when the background palette color 0 is output.
Alternatively you can use any color code from RGBI and/or analogue comparators to generate color keying, depending on what you're after.
I don't know if a 4066 bilateral switch is fast enough for video applications, but you need something like that with a switching rate of ~15 MHz.
