So I tried this setup on a Windows machine, and that does seem to work. So it's a problem in my OSX configuration I suppose, even though the serial_device is set correctly and the baud rate matches.
I did notice this tidbit on the Arduino page:
"The Mega2560 differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it features the ATmega16U2 (ATmega8U2 in the revision 1 and revision 2 boards) programmed as a USB-to-serial converter."
I had already installed the FTDI USB driver, so I'm wondering if that could somehow be causing an issue. I have no idea, though. To be honest, I'm not very familiar with that driver or how I might uninstall it on OSX.