Evidence of rush seating turns up throughout history. The Vikings almost certainly used this method, and in mediaeval Italy simple rush-seated ladderback chairs were commonly used. Strangely, considering that the best rushes grow in England, there's no direct evidence of rush-seating in Britain before the 17th century, though it's quite likely it was used before this.
The pattern is simple, and must have been discovered numerous times by different people throughout history. It's simply a matter of creating a twisted rope of some local material, and wrapping it in a particular way around a wooden frame. You simply attach it to the inside of the frame, and work your way round, over and under, over and under, until you reach the centre of the seat. This produces a strong, but comfortable seat.