Yeah, and I'm not really seeing the advantage of doing this versus just encrypting the data and storing it in normal filesystem space. Maybe that whoever has to write an anti-evasion for it now has to be able to look in this unallocated space on the SSD, and there's no easy API to use?

The malware has to exist outside the OP area to be able to infect the host, otherwise it would never run. Antimalware can just look for that bit. All this really does is let you increase the amount of storage malware can potentially use; it doesn't hide the core, which has to live in the normal area.