Thanks! The "collar" might not need to be as wide as the current pot. Maybe a smaller "chimney" wrapped around the stem would be enough to encourage adventitious roots, which would then grow down and displace existing roots in the white pot. Say, a collar the size of a gallon milk jug or 3-liter soda bottle.
A rather bad idea also just occurred to me. Maybe 2-3 layers of heavy plastic might be wrapped around the outside of the white pot, duct-taped to it. Then duct-tape the seam, and maybe wrap 1-2 bands of duct tape around the plastic (like a belt or girdle).
Then carefully add potting mix, trying to avoid the plastic coming apart and dumping the soil. Probably after filling it with mix, more circles of duct tape (girdles) would be needed to keep it from falling apart.
