I like the idea. I'm not sure about a blanket progressive tax on family wealth, though. I'd prefer taxation stay player controlled. Realm-wide or region-wide control would make for interesting relations.
As suggested, give realm banker the ability to tweak the tax on family wealth and generate money for the crown. Your folks in RealmAia are being taxed too heavily? Take that outrage and get your comrades from RealmBistan to set things right.
Another option would be to give the region lord the ability to set a tax on family estates in their region. Would make positions more sought after, especially those of your kin. Would make heroes more important with the ability to move families.
This puts the money back in the game and makes the location of the 'family home' have more impact. If you want atrophy in the gold supply, make a loss in the transaction. (say, if it's taxed at 5% the crown only gets 3% and the rest is lost in administration).
We need to control this with carrots, not with sticks. Give people stuff to spend their gold on, and they will spend. Arbitrarily take it away from them, and you'll just piss them off.
Although I agree that using the carrot is always better than using the stick on a player, GIVING a stick/carrot to a player (to administer as they wish) is always more interesting and engaging.