@Norilinde: Underwater, yes, about as far as is remotely practical (i.e. not in the deep sea, which will have to await a future tech epoch).
And absolutely, when things get desperate, lots of things will become thinkable to the elites. But how much will be doable, when it flies straight in the face of rules and values you’ve spent 300 years instilling?
The Karagond caste system made itself socially tolerable by setting clear lines about who does and doesn’t merit Harrowing, and for what reasons. If the folks at the top start breaking those rules when they need resources that the rules previously put off-limits, it immediately puts the whole system at risk. It’s the kind of self-destructive decision that regimes often only make when things are falling apart anyway.
I won’t lie: it’ll be really hard. Aetherial blood is the main source of power (economic, military, political) in the world. Other factions both within your rebellion and outside of it will want to keep making the most of blood harvesting. You’d need to defeat them decisively to create a space within which you could lead a blood-free (or only organic-blood, if you don’t mind people being wizards by cutting their own hands) existence without being invaded by people still using industrial-scale Harrowing.
It will be a possible ending. But it would probably take a successful run either as a Genghis-level general or a charismatic Eclect/religion-founder. A high-CHA character can achieve a lot with a cult of personality alone, without meddling with religion… but I don’t think purging the world of Harrowing is an achievable goal operating on Charisma alone, without rolling out a new set of religious institutions.