A whitelist address server is far more scalable than bitcoin. It would also be quite possible, practical, to implement. It could be done today with multisig. You simply have a 3rd party api that you call to get a signature for the multisig send. If the output address doesn't appear in the whitelist, signature denied.

This is the same thing they could potentially do with covenants except it can be done today without a softfork.