If you have an idea, send it to:
Oregon State Capitol
900 Court Street N.E.
Salem, OR 97301-4043
The deadline is October 31st.
Oddly, they don't want art. Just rough ideas, when they will let their artists convert your text to image. (Which ruins my plan to try to hide a pot leaf in the image!)
Actually, Oregon was the first start to legalize medical marijauna, so it might be worthy of being on the quarter. I doubt they would ever select it, but go ahead, give it a try. :-)