yes look like you are really close to the answer... here is what they suggest on their site for a UART, which is essentially a TTL serial port. Basically, you need to invert the data between Arduino and the LCD, which can be done with something as simple as a transistor. Seems like the serial speed is 9600 baud maximum, switch selectable.
D
PS: when you get this figured out, could you post a tutorial of it to the 'playground'? It's useful for others not to have to repeat the same research.
"What about taking data straight from a UART?
The output from a UART is non-inverted serial. It's upside down with respect to the RS-232 logic that the Backpack expects (where a logic 0 is +2.5 to +15V and a logic 1 is -15 to +0.5V). So it has to be inverted before being fed to the Backpack. If you are writing your own software serial-out routines or using something like the BASIC Stamp, just invert the bits you send to the I/O pin that drives the Backpack. If you are using a hardware UART, you'll have to invert in hardware. (If you invert the data bits, the start and stop bits will still be wrong and you'll get gibberish.) You can either use one section of an inverter IC, like a 7404, or a simple one-transistor inverter."