I had reported that Henderson Island is covered in poisonous trees. I was mistaken, that's Easter Island (which is nearby). There are as far as I know, no poisonous trees there.
Henderson Island is a source of wood for the people of Pitcairn, they come to the island 1-3 times per year for lumber. I believe they land at the northern beach (the south and western approaches are cliffs, the beaches are on the north and east), but finding out exactly what their habits are with regard to this would be critical to deciding where to put our settlement. Informant(s) on Pitcairn could be workable, but it's also another link in the chain where the secret of our habitation might get out. (Not to mention all the dumb ass anons who will know - the trick there is to prevent them from being able to prove it/get anyone to care/believe them).
Idea: Introducing bananas trees? There should be lots of information on the distribution of banana trees to the new world, on tropical islands that were much like Henderson Island. They were mistakenly believed to be native to the new world because they had spread so widely, suggesting that we might need to be cautious not to let them spread beyond our needs.