I turned to a method called the Seldinger Technique, used in the nursing/medical profession to insert central lines.
The problem with the armature I use is that the ends aren't smooth and pointy, and therefore get caught up in the polyfil - even if I've made a channel in the stuffing with a knitting needle. So I thought I might use a plastic drinking straw to make a temporary channel for the armature. I found a knitting need close to the size of the straw, so I could slide the straw over the needle and through the stuffing.
After the straw was inserted and the knitting needle removed, the armature slid really easily in the straw.
Then the straws were removed, and voila! Armature in place!
I whip stitched the horizontal part of the wire to the doll to keep it from sliding around, and have covered the wire behind the legs with fabric tubes that match her stockings.