I just joined the Registry. I cannot speak about the Grand Mayan, but for example Pueblo Bonito Sunset Beach, is part of the Signature Collection at RCI/Registry. I bought at the Montecristo the fractional ownership and therefore now belong to the Registry. I can trade my registry Montecristo into Pueblo Bonito Sunset Beach TS weeks at about a 1:6 ratio. So $2,000 of MF gets me 6 TS weeks that as the equivalent MF cost of $6,000. But I cannot trade a TS week into a fractional interest exchange.
In addition, and this is relevant I was advised that if I requested a Signature Collection week on a wait list, because none were in the Registry Collection inventory, the next available RCI signature week would be transferred into the Registry Collection. That makes it harder for RCI traders.
My impression however is there is a lot of units being traded into RCI in all the Mexico Resorts so a scarcity is not realistic. What is more realistic is Disney is now in RCI and I am sure hoping to get a priority through the Registry, but how many people are going to spend the huge developer prices to the trade among RCI units. Most fractional owners buy a particular resort to return there each year or to trade into other high end fractional resorts.