American Express Membership Rewards terms already restrict transfers to the frequent flyer accounts of the primary cardholder and (in the USA) the accounts of supplementary cardholders. In practice this policy is seldom enforced:
It remains to be seen how Chase will enforce their restated policy. I would be surprised if they closed an account due to a transfer to the frequent traveler account of another cardholder on the same account, but the rule does not create an exception for anyone other than a spouse (or domestic partner).
I transferred points to an account which was not an authorized user on the original MR account last week, with no problem.