Actually I think we are talking about two different requirements and their possible solutions. First the voltage/current driver/receiver requirements need to support RS-422 full duplex four wire serial links is easily met with the many TTL to RS-422/485 type driver/receiver chips.
However the requirement for 8 UART full duplex channels is quite a different challenge. The arduino meg boards of course have 4 hardware UART full duplex channels on chip so that doesn't solve the need for four more. I would think one nice solution would be to use the required number of I2C/SPI UART chips (along with the additional driver/receiver chips mentioned above) that are available such as this one:
Lefty