You cannot visit the Square Head. Inside are the offices of the management of the Louis Nucera public library. It is a very unusual building, though, and you will probably enjoy to see it for real.
It is easy to go there. It is not far from the MAMdAC, the congress center and the national theater. Not far either from the Natural history museum, the Garibaldi square an the Republique street (streetcar stops : "Garibaldi" ou "Acropolis").
You're bound to pass not far from it. So why not have a look?