Someone sent me a publication (Rivera & Svenson 2016) in which some photos of ootheca. The "egg" shown above looks quite a bit like the ootheca of a
Hicetia sp. (p. 629, fig. 14k). The oothecas of
Liturgusa shown in the same publication are less similar in my opinion. At least one species of
Hicetia (
H. goeldiana) does occur in neighboring French Guiana according to a list of species on Wikipedia. I'm finally convinced this globule is, indeed, the ootheca of a praying mantis.

The ootheca is normally attached to a twig or leaf, and should have a "seam" on the side which is not visible on the photo.
Cheers, Auke