I have not made down pants, but I have made insulated pants using Polarguard 3D insulation. In dealing with the loft of the insulation, you have to add a suprising amount of fabric to the outside layer of fabric. If you do the math, adding 1" of loft means adding 6.28" of circumferencef or each leg. I did not allow enough at first and had to add a gusset in the thighs and at the hips to make things work out properly.
As to dealing with the quilt lines, my only experience is from way back when a friend of mine made a down jacket from a Frostline kit. The instructions had him fill each panel (front, back or sleeve) with down, and then pin the quilt lines, carefully shifting the down around to get it even, then sew the lines. But pinning an ultralight fabric isn't the greatest idea if you want it to staty downproof. I think I'd quilt it almost all the way, leaving a few inches of each quilt line unstitched, fill each channel, sew the edge closed, then use the fewo inches of open space to fine-tune the balance. The tricky bit would seem to be getting the amount of down you want into each panel. The sources of down I am aware of sell in packets of 3 ounces, and you might want more or less than that in any given panel - of course with pants you can just decide to have 3 oz. in each leg and then it's easier - that would probably be about right for a nice warm pair of pants anyway. I decided not to do down because I didn't want to deal with the added hassle. Sewing with synthetic insulation in batts is much easier. My pants are about 9 1/2 oz and very warm.