> Yeah, but the government would have to prove it in court, which is hard and takes a long time.

Isn't this "undeveloped in perpetuity" status an application, so that you have to request an agreement to your valuation and the government has to approve it, meaning that the burden of proof goes the other way from your comment? At least, for my personal residence where I have the opposite incentive, it's not that I can go to the local government with a valuation of $3.50 for my house and they have to prove it's not; I can object to their valuation and try to prove my case, but the burden is on me, not on them.