The USB ttl pinout for the RBBB is a semi standard, its a adaption of the standard FTDI cable which will also work with the RBBB with the exception of auto reset. A near identical FTDI breakout board is sold by Sparkfun amongst others with a compatible pinout (the unused pin is different to the Modern devices board)
Is it all a matter of where you send the data? I mean to say, if they're both a form of Serial, what's to stop you connecting a Serial cable correctly through the ICSP and burning the bootloader?
This is where I get out of my depth, you'd better ask one of the Arduino developers. I've used both the RBBB and its FTDI board and burned bootloaders using a seperate programmer through the ICSP, I just accept they're different........