I have a Seagate Barracuda 7200.11 here with the firmware bug that holds the drive in a BSY loop.
However, for some strange reason, it doesn't seem to accept any data on its RX pin and the TX data only seems readable with a small pull-up resistor+diode combo.

I thought the serial connector used 3.3v TTL levels and not 1.8v? Anyway, with pullups/pulldowns on the RX pin it still doesn't really do anything regarding receiving.

Connected to a Prolific or FTDI based USB-serial adapter doesn't change a thing, only get the "LED:000000CC FAddr:00249EF5" and nothing else. Repeats every ~60sec.
Ctrl+T, CTRL+Z, no prompt comes up. Any idea why this drive wants to be 'different'? As an extra I could use a native RS232 port with an extra RS232-TTL adapter but I'm not sure this is the problem.

Full label: http://i.imgur.com/u7OZe4b.jpg

PCB ID's:

100466824 rev c (this is etched in the board copper layer)
on the sticker below it: 100468979 j k902bbd7

When I connect to a random samsung drive, serial I/O is fine, when I connect to a different seagate drive, it also works, so I know my setup is working on the serial side.

Anyone have an idea why this board is misbehaving 'extra' on top of the bad firmware?

It's because drive has problems. It is starting, getting problem and stops to answer to ata and to terminal too. You need to try to short read channel between starting and getting problem. It's not easy to find right time when channel should be shorted.