FYI, For anyone who stumbles upon this in the future, the correct messages are:
0x68, <len>, 0x3B, 0xA5, 0x63, 0x01, <index: 0x40 + Index number (1-5)>, <Text>, <CRC>
Then Update the screen:
0x68, 0x06, 0x3B, 0xA5, 0x63, 0x00, 0x00, 0x93
As previously stated, you need an MKIV Nav with software version > 40. Note that you can write up to 38 characters per line in this mode.
Thanks!
-Ted