I also didn't manage to get to the 'Flora do Brasil' also not yesterday they are probably working on it..
I feel you're on the right track with the Custard Apple, I've seen fallen fruits on the ground, but in another area by the river. The flowers though also look so waxy..
Yes, it is very weird that the flowers don't seem to have stamens, another strange thing about them i also noticed at the time.
No I really think it is not a vine, it is a tree, though not that very big, I traced back the rather thin trunk all the way down, amongst all the entanglement, took a photo of it even but the light conditions were so challenging, it came out too blurred. It is very difficult with a scorching sunlight from way above, the contrasts are very strong..
I did found what looked like a seed pod though on the bolder underneath the tree, but wasn't sure if it was from the tree..
The seed pod, but probably from another plant, I think I see some remnants of stamen on them..:
I opened it up but it looks moulded inside..
