The quilting lines on your "inner quilt" may telegraph thru to the dupioni. I would do a small version first using your intended fabrics.

That is a good idea PaperPrincess, thank you. If it does infact telegraph, is there anything I can do about it? I really want it to be as smooth as possible, without resorting to commercial comforter style tacking (ick!!!!!! talk about a quick way to ruin an otherwise beautiful top design)

Now you are wandering down the road of something so heavy you will suffocate under the weight.

Because you would have to line your silk to create a smooth barrier between it and the quilting.

Since you want an absolutely smooth top consider this: instead of a duvet cover create a bedspread (lined) - a bed topping over your sheets which you could pull smooth. Throw your comforter in the closet for the daytime. I have seen some modern rooms with just this sort of look, without loft on the bed.

The only thing I can think of is purchase a blanket instead of batting. I would go with a thinner manmade fiber, then add your top, no backing needed, although you can add one if desired. The blanket will not need to be quilted at all, just cut it to the desired size and bound. The only difficult thing would be to make sure that the top is smooth on the blanket. I would lay the center portion on the blanket and baste around the edge. Lay this in-process piece on the bed to ensure that the drape is correct and adjust as necessary. When you are happy with this part, add your borders with the sew and flip method, trim the blanket to the correct size and bind it.

That is definitely an idea, and something I will consider.

If I went this route, would I take the blanket the full length and width of the top? If not, would the point where it stops be noticed?

Yes, the blanket would be the full area of the quilt. It would be like you were using the blanket as the foundation for foundation piecing if you are familiar with that technique.
Here's a tutorial for a sew & flip:
http://www.diaryofaquilter.com/2010/...-tutorial.html
She is using all small strips. Her first piece is the pink center strip. Your first piece would be the big center square, then add your border strips. When your borders are all added trim off the excess blanket and bind.
Reading your posts it sounds like you are looking for a flat, smooth top portion. With any method you choose, you will need to be careful that the large top piece is correctly matched to where it's sewn to the batting or blanket or whatever you use. If you sew the 2 together and the top is even slightly larger or smaller than where you sewed it to the batting, it's not gonna lay right. Normally I would suggest basting the top to the backing in a grid of about 8" but if you use dupioni you can't do that. Can't baste silk where it's going to be seen because the needle marks don't come out.

Thank you PaperPrincess, you have been absolutely invaluable in offering me many options, as well as things to think about as I continue planning. By chance, if I were able to locate satin in the right color (it wouldn't be wasting money or fabric as I have not yet purchased anything) could I baste, or would I have the same issue with the pin marks not coming out?

Re: pin marks in satin. This depends on the actual fabric content some will show some won't. Also, you do realize that there are going to be seams in the top. Most home dec fabric is about 54-56" wide, so even if your bed is a full a single cut won't cover once you remove the selvedges and add the seam allowance.

D'oh! You are right, the silk I found is 54" wide, not sure why I didn't look at that before. I was able to find 118" polyester satin, they just don't have the silver (had planned on using this for the back). Hmmm, more research. If I have to piece the top, what would be the "cleanest" way to do it?

Hmmm, just had an idea, using triangles to piece it, with them meeting in the center, might work, depending on several things.