I use a Hennessy Hammock (zip model), and I like the undercover approach. To answer the OPs question, you do NOT put your insulation between your body and the hammock -- it goes below the hammock (suspended by the undercover). Hennessy supplies a foam pad that works with the undercover, and it is lightweight but bulky. I got a radiant pad for $6 on ebay which is lighter and is just as warm, plus it makes a great sitpad/lounging pad during the daytime while taking short breaks when riding.

Underquilts are nice, but they aren't cheap (often as much or more than the hammock), and they are exposed to the elements from below -- the undercover/radiant pad combination is a bit bulkier, but bullet-proof and weatherproof, and can be used if you ever have to "go to ground" tarp-style with your hammock if there are no trees. They make a good alternative for some people.