Is it possible to instance a rigged humanoid many times with geometry nodes, and to change the posture of each instance individually ? (A bit like Miarmy is doing with Maya)
No. The instant that it stops having identical data-- like, position, because one guy’s arm is up and the other isn’t-- then it stops being an instance.
As far as armature deformation in GN, it would technically be possible, but it’s one of the most impractical things you could do in GN. It would take a week to implement it and, afterwards, it wouldn’t be any good except for with the specific armature you designed it for.
Just link in a bunch of copies from a different file.