Farkle is generally attributed as being coined by the ST1100 crowd. A combination of functional and sparkle.
Pannier is of French origin, I think. It simply means bread basket. Later adapted into English in the 1200's to refer to side carrying baskets for the horse or beast of burden.
Pillion is derived from an ancient Gaelic term for rug or animal skin. Leather. This morphed into covering for seats. Why it's now just a reference for just the rear seat, I don't know, but the term is used for both seat and passenger.
And you are correct, American biker slang does include the term bitch seat to describe the pillion, but one needs to drop the 'seat' to describe the passenger...
-The Perfesser