So the book of the undead is more specific than the AP player's guide. It says:
"The dead and the quick have certain enumerated rights,
such as the right to practice a trade, to require witnesses
if accused of a crime, to receive recompense for lost or
damaged property, and so forth. The living and undead
are also restricted from interacting with one another in
specific ways: it’s a crime for one of the dead to feed
upon the quick except in certain circumstances, and the
quick are prohibited from using positive energy at any
So I think that means the spirit instinct barbarian should simply become a ghost (a bit of a meta joke for that instinct). Bing bang boom, now you're undead using positive energy and following the law.
Bonus points for dying by Level 2 to become that ghost.
Can't speak for how that excerpt from BoTU is worded, but the couple other sources I found seem to corroborate with how the BL players guide was worded and its intent. That is; The laws of Geb prohibiting the channeling of positive energy within its borders - which is pretty much how the Inner Sea World Guide words it anyways.