This project has evolved lots and still has a way to go. Basically, it is RS485. It has two pairs for full duplex serial RX and TX, with a little half duplex pair (that grew out of DTR) for out of band management.
It is hardware intensive, but frankly, I figure I have more skills with hardware than bootloaders, and Optiboot is working well. I also didn't think hacking avrdude to get a half duplex bootloader working sounded fun.
The latest version is sort of self-bricking, and I figure everyone might find that a little humorous. I don't really understand what is happening but when I set the RX and TX transceivers to pass data through (e.g. from the driver input to receiver output) and upload a sketch to the local Uno that the shield is plugged into it ends up bricking the management MCU. I found it works when I disconnect the jumper (R20) between the Uno SS (pin 10) and management MCU reset. I was hoping to be able to plug the board into an Uno and program the management MCU with the "Arduino as ISP" example sketch. I wonder if anyone else has got a brick after leaving a target connected to the Uno with "Arduino as ISP" and then bootload something else (e.g. changing from "Arduino as ISP" to blink).
The board page has schematics and stuff (it is not a finished project):
http://epccs.org/indexes/Board/RPUftdi/