nvram boot-args to boot in target disk mode
From this (very useful!) page : http://www.cnet.com/news/how-to-manage-os-x-boot-options-with-wireless-keyboards/

I found the answer you want. And I quote:


  The "boot-args" variable can be used for setting the above modes. but in addition if you need to load the system into Target Disk mode, you can do so by setting the "target-mode" PRAM variable in the following way (this will be enabled only once for the next time the system is started):

sudo nvram target-mode=1



It looks like you can't get it to do that on every boot - but you could write a startup script that waited for a minute then set that parameter again then rebooted. Note that without the delay, you would never be able to boot the machine again :)

Then if you ever did want to start the machine not in Target Disk Mode, you would simply kill the startup script while it was waiting for the minute.