How to enter various nvram boot-args

Yes, PRAM is stored on the same NVRAM chip as the firmware variables, but it is managed separately. It actually shows up as a device in the firmware device tree.

HT1242: What's stored in PRAM

When you reset one, you reset the other.

HT1379: Resetting your Mac's PRAM and NVRAM

AFAIK, Apple has never supported direct developer access to PRAM, so it is not documented well (or at all). The functions readXPRAM and writeXPRAM can be used to access the raw bytes, although these days they are just wrappers around IOKIT functions which access the APL,MacOS75 partition in NVRAM.

The nvram command line program only manipulates the firmware variables stored in NVRAM, and so does not let you access the PRAM partition.

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.