I agree with what was stated there. You can make plots for others by including their public farmer key and pool key into the plot as you plot it. Which is how plotting services work. Their farmer/harvester will accept these plots to farm because the farmer public key can be matched to the private key.
You cannot just assign some existing plots to your private key which have been plotted using someone else’s farmer and pool public keys. This is why you cannot sell plots that exist already without selling the entire lot in one go, and giving up your private key and securing the trust of the buyer that you won’t use a copy of the private key to empty their wallet at a later date.
The buyer can check and transfer any rewards they receive but only if the seller doesn’t notice them first.
Since the private key and farmer public key are linked and cannot be unlinked I cannot see any way unless you secure a legally binding contract which would have to work for the duration those plots, and therefore the private key attached to them, exists.
This will change with poolable plots which presumably could have their ownership transferred to someone else.
Not an expert just this is my understanding of how it works, can any experts wade in?