Hi Andrew,
To get to Mapoon, you need to pass through Weipa, which has a mile of channel markers very close to shore and in small boat range that hold many nasty species of trevally, so worth a shot.
Once in Mapoon, launch your boat, go out of the Bay and turn left and head towards Janie Creek, the flats all the way down are great.
Keep you eyes open and everything happens in less the 2 meters of water and less than 10 meters off the beach. Fish the tides as well, no run no fun.
Also just out from the ramp at Mapoon is the best big queenie and trevelly action in the Cape. Small poppers and small stickbaits work well. Kokoda Rodger poppers are deadly., but also keep a BIg popper handy, no different to most locations, hooked and struggling fish will attract bigger GTs to have a look.
Also if you have any Jackall Mask Vibes, take ALL of these. I have also used the FLT Transam 95 for years, it is the BEST lures for the flats slow rolled, all sizes of trevally love them, just upgrade the hooks and fish light (20-30lb) apart from the sharks, you will land 99% of all your trevs over the flats, no bommies here!
Good luck