I think I figured out why the meter uses an odd physical interface: isolation. Right on its face, the meter says it can handle 600V max. You don't want that kind of voltage around RS232 wiring for the safety of personnel and equipment, so I'm guessing there's an opto-isolator in the mix like this:

Isolation would certainly explain why the meter can't generate its own voltages for signaling (excepting the use of photovoltaic isolators, which are most often seen in low-speed MOSFET drivers).

Recent Updates

I made a few updates to the code and case last night, so if you cloned the repo, you probably want to do a pull.