Can you take the first morning flight out of ITO back to HNL?
Stay at some cheap hotel in Hilo, then drive to airport and you'll be at your conference in the Hyatt by breakfast.
It really is the only way that you can enjoy this "day"trip.
P.S. The Hyatt itself does not run shuttles to the airport, but Roberts' buses are ubiquitous and I believe that they stop at the Hyatt. The concierge can give you a schedule and probably sell you tickets. (Ten bucks o/w, I believe).