Originally Posted by
Trakhak
Bad idea, if the flat Presta nut prevents the rubber at the bottom of the valve stem from sitting flush on the rim's inner surface. The inflated tube will try to fill that gap, possibly stretching the rubber thin enough to fail.
Some 40 years ago I had some Wolber tubes where the presta valve was held onto the rubber tube by a screwed-down nut, rather than being vulcanized/bonded onto the rubber. They would work for a bit, but eventually all became self-flatting as the rubber stressed around the nut.