they've got it already, they can sign any data and put it in their block reward transaction.
And this fact immediately offers to solve many applicative, including technical problems. In what applications is this already used? After all, you can do interesting things with this feature.
For example, sub-chains without pre-mining, but with PoS, in which coins are also mined by bitcoin miners, in case they freeze their bitcoins in these sub-chains for a certain number of bitcoin blocks (the lifetime set by the protocol of this sub-chain), on some kind of smart contracts.
I wonder if this has not been implemented anywhere yet.