Thanks for bumping a somewhat older but still useful thread, Gdwelker! Interesting question!. It certainly won't hurt. The only thing is: your down UQ is probably a differential cut design, right? Meant to be snugged up solidly against the hammock(most are)? If you have it really snug, it will compress the open cell foam pad, and you will probably loose some of the pads insulation. But, I'm sure you will still pick up some more warmth, I'm just not sure if you will get all of the warmth of the pad. Sounds like a good experiment!
However, if you are using the entire HH system with the under cover and/or the space blanket, with your quilt wrapped around all of that, with the UC and/or space blanket working as a vapor barrier, your body vapor will be prevented from traveling into your down, contacting the cold shell or cold outer down, and then condensing into liquid water. This will keep your down much drier so that it will always be at max loft. It will also totally shut down evaporative cooling off of your back side. These together- even without the pad- will probably add 20*F to your warmth, with the added benefit of keeping that down dryer for longer trips. The pad may add even more.
Another way to go-as seen in the above posts- is to simply add that UQ inside the under cover, and under the pad and space blanket(on top). Just lay the UQ in the under cover, no suspension needed. Either spread out wide- single layer- or double layer, but still wide enough to cover your body. This will add the wind and rain/snow protection of the HH UC to the UQ. No worries about wind chill! Some people have already used such set ups way, way below zero F and been toasty. But just pay around and experiment, and you will figure out what works!
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