I can't see why not. A government could require exchanges to prevent (or even just delay or tax) the sale of Bitcoins that originate from miners that cannot be

I can't see why not. A government could require exchanges to prevent (or even just delay or tax) the sale of Bitcoins that originate from miners that cannot be shown to have used renewable energy to produce the block. The government could, for example, physically inspect the mining rig, certify it as renewable-powered, and require that the miner uses a particular keypair to sign their blocks in order to identify them as "green." Exchanges use this (government-issued) keypair to determine which Bitcoins are "green," and apply whatever restrictions are necessary on those that are not.