I've no idea how the govt would be able to do this in a workable way though - how to you value the appreciation in somebody's house when the only meaningful way

I've no idea how the govt would be able to do this in a workable way though - how to you value the appreciation in somebody's house when the only meaningful way to value a property is to sell it? A bureaucrat called a 'tax assessor' makes an assessment - which may or may not be fair - of your property's current value. You're then charged a percentage of this assessed value in taxes. That's how property tax works across America today.