I don't know about old paint brush bristles. Maybe so. But the technique using a BIC lighter might cause a bonfire if the brushes were used with solvent based paints...WOOSH! Why not try it and find out. My paint brushes are too valuable to me to use as weeds on the layout. I would need several thousand brushes to do what I really need anyway.
Let us know if this works.
____________________ It doesn't matter if you win or lose, its' how you rig the game.