Sounds like a great way to have larger competitors with more capital bid for your site to take down your domain that your users were using. Rather than Land Value Tax a better analogy might be a Vacant Property Tax which only targets squatting.
That way you would be forced to keep it personal.. or suffer reduced sale price, as the new owner would need to backpay the minimums you were exempt from, or take on some risk merely leasing from you until the lock eventually expires - as you remain the true owner and all the payments and paperwork must cross your desk.