Hey,
I haven't fished the Yasawas but just returned from a trip to Kadavu, report posted a week and a bit ago.
Popper fishing, we fished the reef edges, casting into the swell basically where it breaks on the reef. Kadavu has a large barrier reef called the Astrolabe and this is where the GT hold up, crusing the edges looking for hapless prey.
Looking at Google Earth, the Yasawas don't seem to have quite the same structure, though down towwards the southern islands like Naviti and Yanggeta there is some interesting reef structure. I guess it would be surprising if there were not GT there, but size and numbers is anyone's guess.
Part of the challenge may also be finding a skipper who has enough knowledge to put you onto the fish that are there. It seems that many charters in Fiji, and similar places, are more focused on trolling for pelagic food/sports species like tuna, marlin, mahi mahi and mackerel. They may well be willing to pull up to the reef and let you cast at it, but knowing where and why the fish hold up where they do and the times to fish for them is as you know critical.
Good luck with it. If you decide to try and jump on a charter elsewhere in the Fiji Islands this time or some other time, I can't recommend Adrian's services at Matava Resort enough.