I release my forestay and tie between mast
and headstay (cutter rig). Since I have hanked on sails
I just attach the forward end to the stay and halyard
for adjusting height. See avatar.
I do find that a line tied between the port and starboard lifelines
just below the hammock is useful for grabbing to damp the swinging resulting from power boat
wakes. That may not be needed for catamarans though.
Spreader bars are a matter of preference, and I prefer to not have them. I find it much more comfortable without the forced flatness. It's a little difficult to explain, but sitting in a free-form hammock at a slight angle doesn't lift
the lower leg and put pressure on the knee. YMMV