Does it like 500kbps?
Nope. Apparently only "standard" bitrates. I can't tell whether it's a MacOS thing or an FTDI driver thing 
(FTDI claims to support non-standard speeds, but when I look for details, all the info is about the windows driver(s))
avrdude -carduino -p atmega1284p -P /dev/tty.usbserial-AM01QQUY -b 500000 -t
avrdude: ser_setspeed(): tcsetattr() failed
avrdude: ser_open(): can't set attributes for device "/dev/tty.usbserial-AM01QQUY": Invalid argument
ioctl("TIOCMGET"): Bad file descriptor
avrdude: ser_close(): can't reset attributes for device: Bad file descriptor
avrdude done. Thank you.