zPilott, that's great that you're getting back into this interface. I remember reading about your first serial version. With you and jimbonzzz doing V1 software, it can only help to inspire more hardware development. I'll read up on the USBMOD3 and see if I can contribute anything.
EDIT: Also, according to Jim's schematics, a resistance value of at least 3.6k is necessary to turn off the V1's display and send the display signal to the concealed display if you don't have one already hooked up. If you do, you should be able to use an RJ11 splitter and forego the need for the resistor loop between +12V and the data line (looks like you've got a 2.2k between +5V and data).
This USB interface looks promising (and simple to boot!).
EDIT2: Is your diode facing that direction intentionally? If I'm reading it correctly, that's the RXD line, and if you have the cathode on that side, wouldn't it be blocking the signal?