Steven et al...
I'm sure that chain hangers were a product of the earlier 'fixed' age rather than a by-product of derailleur gears. This (previously) unrestored Bates from 1935 had one fitted apparently from new presumably to ease turning the wheel round to alter ratio's or change from fixed to free....
http://www.flying-scot.co.uk/
See the second image - the edge of the button head for the hanger can be seen on the inside of the seatstay just above the paint/chrome transition....
I'm certain someone will have an earlier example.....
Bob Reid Stonehaven Scotland