The short version is : pull power from the box, setup Z-modem to send the new firmware file, turn the box on. That's it -- no Tridge style hacking required.
For those of you who missed the BBS experience and skipped straight to the world of TCP/IP, in more detail I did
$ wget http://www.kristal.com.au/opentel/downloads/ODT4200PVR_A_KRISTAL_V202.zip $ unzip ODT4200PVR_A_KRISTAL_V202.zip $ stty -F /dev/ttyS0 ispeed 115200 ospeed 115200 $ sz -vvv -b ODT4200PVR_A_KRISTAL_V202.pgm < /dev/ttyS0 > /dev/ttyS0 [walk over to box, pull out of power point and put back in, it downloads software and reboots]
sz is in the lrzsz package in Debian.
Running strings over the firmware doesn't uncover anything extraordinarily interesting; anyone else pulled it apart any further?
Update : the latest release appears to be a semi-public 2.04 version which can be downloaded directly from Oznetics.