And Dukes collect taxes from Lords.
I don't think it's so very unreasonable if the taxes of someone who gets exiled get confiscated entirely by the person next up the chain.
The problem with that train of thought is that the entire point of the new estate system is that it emphasizes how each portion of the hierarchy is important. Rulers couldn't just confiscate taxes from their Dukes. Dukes would be the ones giving taxes to the rulers. Sure, Rulers can ask for higher taxes, but that doesn't mean they automatically get the gold.
I like the idea of simply making them bonds as it is and preventing the exile from gaining the gold. That way if the ruler wins, the bonds go to them, but if the exile wins they keep the bonds.