I saw the max rate as I was reading the PJRC website. For the serial ports the table read what acceptable baud rates were. So I took that to mean you could not go beyond that.
Fortunately I was able to write a test program which spit out a buffer of ten 0x55 bytes to the port, then measured the end of the final stop bit with a scope. that's 100 bits in all and it came in at 109.5us, giving a baud rate of 913242. Which is only .07% off. So I'm happy the devices will be able to communicate over 422 with this bitrate.