If it's possible, it would be something you would need to turn the machine on and then enter OF mode to set up, then probably hope that the PRAM contents aren't lost, since that seems like the kind of thing that would be forgotten/cleared with a PRAM reset.
The stated purpose of SCSI, Firewire, and ThunderBolt Target Disk modes is generally for speedy file transfers and temporary troubleshooting, not to, say, use a G4 tower as an external hard disk box for another computer, which would explain why Apple didn't bother making it so you could semi-permanently declare TDM.
Depending on what you're looking into doing, it may be worthwhile getting an external disk enclosure to boot from, in lieu of TDM.