The Hennessy in Bivy mode isn't bad provided that the following is true:

1) You use small sticks/poles of some sort to keep the mesh supported above your face. Since the only weight the stick is supporting is the fly and the mesh netting then the sticks don't have to be that strong (ie. not capable of supporting your body weight)

2) Unless you like sleeping directly on hard ground you will likely want a sleeping pad that allows you to be comfortable. For this reason I still take my thermarest along just in case instead of a lighter weight insulating alternative.

The other potential concern might be the bottom entrance for getting into the hammock. On the ground this can be a bit more awkward to get into and possibly more difficult to exit in an get out now type situation.

It's too bad that you don't live close to me. I'd let you give the thing a try which is probably the best way to answer your sleeping question.

~Jamie N