Yes you can build one. GamesX has the pin out for the MD2 A/V port (although the numbering is backwards). You need to connect the R, G, B, Audio (both), Ground and 5V pins to the appropriate pins of the RGB device you want to connect to. I think you'll also need three 75ohm resistors on the RGB lines as the MD makes the signal too hot for standard monitors.
All of these things are no real trouble. You may run into trouble finding the special 10pin mini-din plug that the MD uses for A/V - I haven't found anyone stocking them yet, even the big companies - Farnell and RS Components. That's not to say you won't be able to find them.
You could always butcher your MD and make a new, more standard connector by soldering directly to the mainboard in the right places.
EDIT: Ignore most of that - I was writing it from memory. Check out this thread for more detailed info.