Thank you, all, for the suggestions. I have researched each on the internet and several are appealing. Biriba, Jaboticaba and Camu Camu in particular are ones I will pursue as they can be propagated by seeds that are available by mail order. I can't bring in vegetative plant material to Fiji without a big hassle, but can probably get a biosecurity permit for these seeds.
I already have some Malay Apple (Syzygium malaccense) started and will try a couple of those in the wet areas too.
The pond apple rootstock scares me a little because it is an invasive weed in Australia.
Planting on mounds is a good idea, but due to the expanse of the areas I will be planting, I think instead I'll build a few drainage ditches and use the dirt to build up the surrounding area a little. So much work!
If there are any other ideas, I'll be watching this thread. Thanks.