If someone figures out that you're using the proceeds of that financing towards the war effort somehow

That's a big part of what I find so unlikely. Why would the oligarchs spend any of their wealth on the war? They have almost nothing to gain from it, and the Russian taxpayers (who they also have robbed) are paying for it.

Like I would love to see the oligarchs face justice, but it really doesn't seem like getting at them through the war makes any obvious sense. What they did wrong was rob the Russian people while western democracies turned a blind eye to it for over 20 years while happily taking the money.

Also, how does it even help Ukraine? I think there is some western misconception that rich people have the kind of power over the Russian government that rich people have over Western governments. Putin is a freaking dictator. He can kill the oligarchs if they cross him.