Ok, there is difference in the UART boot management (I think that I discovered a bug in my fimrware).
That maybe explains why I cannot enter in Debug/Boot mode with UART (device is stopped) on my Dockstar.
Did you get any further with this ? I, too, cannot enter UART Debug/Boot mode - but this with my SheevaPlug. 88F6281 A0. Bootrom version 1.11. I cannot see which firmware version you have.
The system just seems to stay "stopped". Anything I download, whether it be a valid bootrom/xmodem signature or just a load of nonsense, just stops the system. Nothing comes back up the line.
I have noticed that the voltage on the TX line seems to stay at its start-up value of about 3.1V and never moves on. I think I expect it to hit 3.3V when the UART switches in, and I am having a thought that perhaps the 6281 is failing to switch the pin from its start-up sampling role over to UART use. But I don't really believe it, I expect I have made a mistake somewhere.
And then I discovered your post. Perhaps there is problem with the chip after all...