Because the poles go inside the tent, there will be nothing to elevate and separate the fly from the canopy. Also, because silnylon expands and therefore sags when the temp drops, as it usually does at night, and from what I understand, NanoShield does not, the silnylon will be clinging to the nano. Since the nano is somewhat vapor permeable, there will be moisture passing through it to add to the cling. So, you will have created a moist vapor barrier, and IMO will have tons of condensation inside.
If you can think of a way to suspend a tarp over the tent, as some have mentioned, then you might have something. Like maybe a rectangle pitched with an A-shape, with 4 corners pegged out beyond the 4 corners of the tent, and supported by trekking poles to create awnings at the front and rear. You'd have to create a scale model dome frame, and experiment with swatches of silnylon to see if you could get a taut pitch somehow. With lighter Cuben, it would be even more difficult, as it does not stretch. You'd also have to deal with the wind stability of the tarp.
And when you get it all done, a lot of weight will be added - the sil fly plus many additional pegs. Or, you could stick with what you've got, or look at the MH Direkt 2 or some of the other lightweight domes coming out. It might be interesting to put high quality carbon poles on a Direckt 2 - result under 2.5 lbs packed. The rain pours in when you enter or exit; but it sounds like that's not an issue for you.