Also, there is a forum here called the sightings board. Posters here will post what exchanges they can see, as well as if they are in the Marriott preference period - Marriott owners have first dibs, then Vistana, then everyone else. It also serves as a historical log of what units were available for exchange, when they were available, and if it was a bulk deposit (meaning that the developer deposited a large number of units) so that you can plan ahead for next time you want to request an exchange. You will be able to see it if you join TUG and pay the $15 membership fee. It is well worth it if you plan to be exchanging. Once you join you will also have access to the TUG review section which could help you choose an alternate resort in case the others don't come through.
Keep your RCI request in - hopefully a Hilton unit will come up again (although I think that was probably part of a bulk deposit that was reported in the sightings forum). Summer is just a tough time of year for Hawaii.