If it's a pill box there should be the gun/observation ports, stick your tree straps through one and back in another. Repeat at the other side and Bob's your uncle
Yes, but I was thinking that the ports might be narrow and maybe spaced too far apart for that (unless you have very long straps), and it would also put the hammock right in the path of the wind coming in through one port and out through the other.
Could work, though, and I'd recommend taking a few small pieces of pine corner moulding to prevent the straps from chafing on the concrete.