I had been mulling on this today, and then discovered your post.
I have obtained the mating 4 and 6 pin connectors and some precrimped sockets
Would you be able to provide part # ?
Is there any use for a RS232 serial connection on the Sheevplug?
Not quite an answer to your question, but a direct connection to the UART might just provide an alternative way of booting the plug from scratch in the absence of OpenOCD. OpenOCD did not work properly on my Mac PPC when I last tried it, due to some unfortunate big/little endian issues in the code. So I am reluctant to proceed with OpenOCD right now. The plan would be to exploit the BootROM Firmware Boot Sequence as described in Marvell's "88F6281, 88F6192, and 88F6180 Functional Specifications" document. (But it might turn out easier to audit the OpenOCD code.
... J6... This is obviously designed for a TWI or I2C bus connection...
One possibility would be to use the I2C to connect up a simple low power LED "status" display. Ideally one that could be powered off the I2C power line (3.3V might be a problem). I did notice that some pre-built modules have an I2C interface built in. There appears to be I2C support in the kernel, although for a different board. Perhaps it could be tweaked into action for the plug.