Thanks Kitsch - you are spot on in your description and what I'm trying to achieve - it's the basic mechanism that early Roland xox boxes used before midi.
You say the pulse/trig from the external device should hit the clock pin on the link port - cool, first mystery solved - in line with these instructions I found for a different project :
"Splice a Game Boy link cable. The green wire (clock) goes to Freeduino pin 23 (in fact, you can sync up to 6 Game Boys running LSDJ with this device... simply connect clock signals to Freeduino pins 23 - 28). Connect the blue (ground), orange (serin or serout) and red (serout or serin) to ground. You may need to make more ground connections by bridging the ground bus with another row."
http://little-scale.blogspot.co.uk/2008 … e-boy.html
So I splice my pulse carrying wire to the clock wire on the link cable - now I just have to figure out how to eliminate the ground/noise issue noted in the original video comments. Seems like littlescale quoted above has everything else going to ground...