I found a fair bit from googling about, including an outdated mostly-correct 2008 command listing from Samsung themselves. Their site also has a newer 2009-2010 command spreadsheet, but it is totally inaccurate for the 2010 set I have here.

The DB-9 on your set may have the same kind of interface, and some older sets even had info in the user-manual for this stuff (no more, though).

Along the same lines.. if I power the set off, and then enter this sequence on the remote control, it gets me into "service mode": MUTE 1 8 2 POWER
Other samsungs had a difference sequence there.

Once in the Service menus, the UART can be changed into "debug" mode (as opposed to "uart" mode), whereby it becomes the Linux boot console for the TV (115200bps). My set appears to not accept input from the port, only using it for output. For earlier years, it was possible to login and gain control of the internal ARM system this way.

Cheers