I don't think they can do it, simply because it devalues the entire offering for all - plain and simple. It's the old adage cut of your nose to spite your face.
One of the incentives for buying DVC is the fact that it has value to others and therefore can be transferable, providing a reasonable rate of return.
By restricting some of its customers for wanting in at a lower rate (i.e. re-sale), Disney will be highlighting more of the obvious; the value just isn't there right now for their product.
If the value was there Disney would be exercising their first right of refusal left right and centre, but clearly they don't, because the value is not there.
Strategically, should they do this, I see this as short term move of stupidity, again to the detriment to all points holders, present and future.
For example it would be like an auto dealer who entices you to buy their vehicle because it offers incredible resale value. But then you find out that in fact the value isnt there even the dealer doesnt want to buy it back from you, and if actually CAN find someone else to sell it to the dealer wont honour the original warranty or service agreement simply because theyre being petty because they cant enough new vehicles off of their lot. YES that prospect really entices me to want to buy (sorry just a little sarcasm there)!
In truth if DVC did such a thing, I certainly wouldnt buy direct they would be untrustworthy in my opinion.
Moreover if people think the re-sale point market is overselling now I say just give this rule a chance, watch the prices drop, and drop hard in my opinion. Which again in my opinion, makes Disneys job to entice direct sales even harder. Everyone loses. The new magic of Disney?

Sorry just kidding around.
Seriously though, I see such a move turning into a vicious circle ... almost deflationary like to the entire point market both re-sale and direct.
Sorry dont mean to be Mr. Doom and Gloom.
Cheers,
Zebsterama