Originally Posted by Rain Man
Or if that's not an option, he could put a cross-brace between two joists (on each end of the hammock) and hang from them. Eliminates torque, spreads load between two joists on each end, and you can drill, screw, nail, and modify all you want, without touching the joists (except to fasten the two braces).
By "cross-brace," I mean a board cut to fit perfectly between two joists and fastened in at 90 degrees to them.
I did a combination of what earlier folks said about drilling a hole for Amsteel (and used a countersink bit to round off the edges) and what Rain Man said about cross-braces. I found the single joist wanted to flex sideways if the hammock swung (or so I imagined), so I added two cross-braces at each end to tie the joist to its two neighbors. Works great.