Here's the gen/md 3-country mod diagram.
after some messing around today, I found out a genesis model 1 can be modded with a simple 3x flipswitch WITHOUT the need for anything else but the switch and wires.
this has been tested on a model 1 european MegaDrive, but should work on other model 1's as well, and possibly model 2+3 too.
buy a single pole (3 pins), triple throw (up, middle, down) flipswitch.
cut JP2 and JP4, if they are connected between the 2 spots (which is the case on PAL machines).
now connect the left side of JP2 to one side pin of the switch, the left point of JP4 to the other side pin, and connect the middle pin to any ground spot on the board. the large silver part at the back of the mainboard would work, but also the right side of JP1 or 3.
In order for the system to run at 50 Hz, JP2 needs to be grounded.
to run in japanese mode, JP4 needs to be grounded.if they are left open, it runs at 60 Hz and/or english mode.
with the switch in the top position, JP2 is grounded and JP4 is left unconnected, resulting in 50 Hz english mode = European PAL mode.
with the switch in the bottom position, JP2 is left unconnected, and JP4 is grounded, resulting in 60 Hz japanese mode.
with the switch in the middle position, both JP2 and JP4 are unconnected, resulting in 60 Hz english mode = US mode.
I don't know if the same works for Model 2/3 and Nomad, if someone tries it out, report back here please.