Since it sounds like you will be wading as well - do not forget Lycra shorts to wear beneath - chaffing will be a massive and painful problem without it. Good wading/reef shoes that go well above the ankle.
Warwick is correct about the lure front but also take some cheapies since you may cast landbased off reefs.
Pre-made leaders/spare spools.
Waterproof bag for cameras and other items you can't afford to get wet. Usually sealed by rolling the top shut.
Do the tenders have GPS? Suggest taking a handheld of they don't.
Water-proof lipbalm and zinc to cover the face (and or buff).
Sounds like an awesome trip Serge.. Very jealous!!! There has been alot of exposure about St.Brandon here in SA recently, all by the flyfishing guys as apparently it is Bonefish Mecca.... better than Sey even, and having fished Sey, if its better than that, you guys are in for an incredible trip!!!
Like Luke says, the lycra shorts do make a huge difference... feels a little strange and camp donning a pair of skin tight pants in the missle of nowhere, but your nether regions will thank you!!!
Also cannot stress the need for high quality wading boots with good ankle support... bite the bullet and get yourself a pair of Simms Flats SNeakers or Patagonia Marlwalkers or similar.... Boots and footwear take a HAMMERING on the flats!!! Don't even think about anything but solid boots... one little coral cut or rolled ankle will end your trip very quickly... get the Gravel Guards and decent socks to go with them too... Also make sure you wear your boots for a couple weeks before the trip.... just like they say you shouldn't run a marathon in a new pair of running shoes, same goes for Wading boots.... I was given this advice and didn't really heed it, and my legs and feet suffered the first two days.... once worn in, my boots fit like gloves though...
Bob Marley is a much better choice than Jimmy Buffet, I agree!! :-)
A few other things to suggest:
Good quality ziploc bags to keep small things dry on the flats
Take a fair amount of soft tissue papaer ( white gold ) or a bandana and keep it dry in the ziploc bags to use to clean your sunglasses while wading... nothing worse than having misted salted glasses and nothing to clean them with or wipe your eyes... its not called white gold for nothing!!!
There is a good chance you could run into Permit out there too, so make sure you pack some Velcro Crabs, Permit Hermits, Fighting Crab patterns for the long rod too...
The sun on the flats is INTENSE, I cannot stress this enough... there is ZERO shade, unlike on a boat where you can find a spot of shade for a few minutes... So be sure to carry plenty of liquid in a camel pack and preferably filled with some kind of rehydrating/isotonic juice of some sort... drink as much as you can...
Also be sure to wear appropriate clothing.... take the highest factor waterproof sunblock you can... and reapply often... Try get one of the brands designed for surfers, its super strong and waterproof.
A buff/Bandana and quality hat with a dark underbrim are essential...
Sorry if I'm stating the obvious about the sun/heat/water, but its important... I lived in the Caribbean for 5 years, running a charter boat, and thought I was used to the heat ( grew up in sub tropical SA east coast too ) but Seychelles kicked my ass pretty hard.... in fact got so dehydrated and buggered the first session sight casting GT's in the surf, I walked back to the little island/atoll to try rest in some shade in a cave... my WHOLE body went into such intense cramps I couldn't move for 45 minutes.... and the tide was coming in rather rapidly.... thankfully my old man found me by the time the tide was at my waist and dragged me out....
DRINK LOTS OF FLUIDS!!!
Have a great trip Serge, sounds like its going to be incredible!