Maybe - I'm guessing - water the area well 2-6-10 hours before flaming. Maybe spray intermittently for a long time, so the leaves get well soaked without needing inches and inches of water.
Most of the water will run off the weeds or evaporate over 2-10 hours. Then flame only long enough and up-high enough to "steam" the weeds without drying out the leaves underneath.
I never tried this and didn't even "see it on the Internet". Just speculating.
