So here it is, the pattern I wrote for Rinoa Heartilly's duster. The PDF also includes a bonus pattern for the arm warmers.
There may still be mistakes in the pattern; if you find one, please let me know! I'd also be very happy to see pictures if someone does knit this. Please only use this pattern for personal, non-commercial use. The pattern can also be found on Ravelry.
Final Fantasy And All Respective Names are Trademark & © of Square Co., Ltd, 1997, 1998.
Pattern updated 21-09-2012
EDIT: Apparently, this deviation is responsible for 25% of the views of my gallery. That's pretty amazing, and quite inspires me to make patterns available in the future!