I tried out this mod in my latest DMG, and it works pretty well. I needed to send the modular clock into an envelope first to get it to trigger the Game Boy which was a bit odd, but it syncs, so no big deal.
I did try to do the mod without a switch, but couldn't get it to work. Clock Out works when both in/out are wired up at the same time, but not In. As if LSDJ is getting confused with the pulses. I don't see why it shouldn't work though, so maybe worth someone with more electronics knowledge than me to have a bash. In the end I went with a teeny tiny DIP-style switch that is recessed in the case just above the sync jack.
In the future I might investigate putting the sync jack in the top right so that the cable won't get in the way of my right hand as much.